"What I wish someone had told me about high school": ever wish you had a cool older sister who would tell you everything you needed to know to make high school the best time you've ever had? We got 17 seniors to spill on what even your big sister doesn't know.

The summer before my freshman year, I had dozens of expectations of what high school would be like. I would be a straight-A student and reigning member of the popular clique. I would have a hot boyfriend with a great car, get elected class president, land the lead in all the school plays and be a star varsity lacrosse player. Never mind the fact that all my grand ideas of high school life were based on teen flicks and WB dramas. I was absolutely convinced this was how high school would really be.

It only took the first week (and three near nervous breakdowns) to figure out I was dead wrong all along. The reality was that I had no clue how to deal with changing friendships, massive amounts of homework, finding my niche at school and, yes, even getting down with the whole dating-and-guys thing. Now that I've finally graduated, I can't rewrite the last four years of my life. But what I can do is give other girls the advice I wish someone had doled out to me and my friends. No doubt, making your own mistakes and learning from them is a key part of surviving the high-school scene. But here are 18 lessons my friends and I learned the hard way ... so you don't have to!

#1 The popular group isn't always the best group.

When I was a freshman, I asked my friend Joanna how I could fit in with the popular crowd. Her advice was to stop using so many big words, stop thinking so much and to wear more makeup. Needless to say, I never quite fit in among the small-word, no-thinking makeup masters. Instead, I found a group of five girls who are now as close to me as family.

Having a crew of friends who accept you for who you are is what makes high school memorable. Maybe your best friends are in the popular group; maybe not. Either way, having really close friends who will be there for you through thick or thin is more important than anything. As my friend Sam says, "Knowing you have people you can call in the middle of the night is the greatest feeling in the world."

#2 Trust your gut.

My friend Lindsay recalls, "I made a decision not to drink during high school. There were times when I thought to myself, 'Screw it--everyone else is doing it, so why shouldn't I?' I never acted on it though. I don't need to drink to have fun or to base friendships around. If it's not in your nature to cave to peer pressure, don't."

If you feel like something is wrong, then it probably is. Even if your friends are telling you it's the right thing to do, that doesn't always make it right. There have been situations in which I was torn on what to do, and I made decisions based on what others were doing--even though it went against what I felt. Wrong call. Everyone makes mistakes, and everyone uses bad judgment at times. But listen to your gut to avoid making mistakes you'll later regret.

#3 Upper-classmen are not as scary as you think.

The first few days of high school, seniors might try to make you feel their "power." Don't panic--it's a high-school tradition for the upperclass to intimidate freshmen! Getting to know upperclassmen through sports teams or other extracurriculars can be really helpful when you're trying to figure out details like which English teachers to avoid or how to cut detentions. That same senior you feared could ultimately turn out to be a really good friend and mentor.

My friend Shannon got in with upperclassmen on her volleyball team: "I became really close with my older teammates. If you are a dedicated, excited team player, upper classmen respect you. It's a great way to make cool older friends who can help you out."

#4 Do your homework.

First semester of your freshman year is kind of like a warmup for the rest of high school. As you progress, you'll be given a lot more work and expectations will be higher. So adopt good study habits now, because procrastination will catch up to you ... and quick.

"I am the world's biggest slacker, no joke," my friend Katie confesses. "I put everything off until the last minute, even if it's been assigned five months in advance." Don't procrastinate! Break big assignments into small chunks you can tackle each night. And beware of time-wasters. I was big on checking celeb's AOL profiles or whatever when none of my friends were online--so stupid. Work hard on your own and, not only will everything be less stressful, you'll be rewarded with great grades and the ability to survive college!

#5 Let people change.

"Do you really want to be the same person you were in middle school?" asks my girl Lauryn. It's good to learn early on that nothing is permanent. As you and your friends have new experiences, take on fresh activities and get acquainted with different people, you're all going to change to some degree. Change can be positive for everyone in the long run--even if it is scary at times. Being a good friend means supporting your pals' hopes and interests. Even if those hopes and interests don't always include you.

#6 Boys aren't everyting.

Yes, boys are cute ... really cute. But never let the quest for a boyfriend take over your life. If you have a cool BF, great, but you can be just as happy without a guy. As my bud Beccy remembers, "I didn't need a boyfriend to make my high-school experience complete. Whenever I had free time, I wanted to be with my girls. Having a boyfriend probably wouldn't have let me experience high school to the fullest because I wouldn't have been able to do everything I wanted to, when I wanted to. Not to say I don't ever want a boyfriend or that there weren't times when I did want a boyfriend, but it hasn't been essential to my happiness for the past four years." Boys are only a big deal if you convince yourself that they're a big deal.

#7 Find a passion--and follow it.

Kelly says, "There are so many things in high school I wish I'd done. In a way, I regret not doing certain things I enjoyed doing. Finding what you're passionate about is important, but even more important is listening to that passion and making it a reality." Get involved in activities and classes that allow you to learn more about the things you love. If you let opportunities pass you by, you'll always ask yourself, "What if ...?"

#8 Don't waste time worrying about what others think.

No offense, but you'd be surprised at how infrequently other people actually think about you. Girls aren't always nice; in fact, we can even be horrible to each other. But while gossip is a natural part of high school, don't let what others say determine how you see yourself.

Says my friend Michele, "I spent so much of high school letting other people dictate what I wore, who I hung out with, what I did on the weekends. I was never satisfied with myself or my life. It took awhile, but I was able to block out all those voices and listen to myself and what I wanted. Now, I'm much happier and more confident."

#9 Be yourself.

Cliched, yes, but it really is true. Everyone has quirks--that's what makes people unique. There's nothing interesting about a high school full of clones. Embrace your uniqueness. "High school is a challenging time. Everyone is changing and experiencing new things. To enjoy your time, always be who you are," advises Elizabeth. "Opportunities are going to present themselves, and who knows where these opportunities might lead? If you worry about how it might 'look,' you'll never find out. Don't ever change who you are just to fit in. You won't be happy."

#10 Your reputation can follow you throughout and after high school.

Remember that girl who walked out of the bathroom with her skirt tucked into her underwear? Or the boy who burped out loud during a silent moment in assembly? Of course, you do. We remember people by their actions as much as we remember their hair color. While embarrassing moments are unavoidable and nothing to be ashamed about, dishonest, vicious or sleazy behavior is well within your control.

Take responsibility for your actions, and choose them wisely. It's way better to be remembered for being an awesome lacrosse player or a straight-A student than for being the girl who hooked up with three guys in one weekend.

#11 Don't let others' expectations stress you.

You'll be met with lots of expectations from parents, coaches, teachers, and even friends and sibs. But if you're doing what makes you happy, in the end, people should be satisfied with that. No matter how much your mom wants you to be a cheerleader, if you'd be miserable doing the rah-rah thing, what's the point?

Sydnee had this experience when she entered high school: "When I was a freshman, I'd been playing lacrosse for eight years. I'd dreamed of playing for my high school team and then for college. But by spring, I realized lacrosse wasn't as much fun as it used to be. Soon after, I got involved in theater. When I was onstage, I felt the same excitement I once felt when I played lacrosse. A lot of people criticized me for my decision to give up lacrosse to pursue theater, but it felt right to me." Don't do the things you've always done because people expect you to. Now's the time to seek out new experiences instead of relying on what's safe. Dive into something new, and give it all you've got.

#12 Avoid freshman burnout.

Even if you ignore our previous advice about keeping on top of homework, remember this: Don't pull all-nighters for a week to get the work done. There will be times when it seems like every class has a huge project due. If you're feeling overwhelmed, talk to your teachers. Most will be cool and give you an extension, as long as you have a valid excuse. No, "I was watching an O.C. marathon on Fox" doesn't count.

#13 Mistakes are OK.

Everyone trips up every now and then, even teachers, coaches and parents. It may not feel like it, but nobody expects you to be perfect all the time. The important thing is to learn from mistakes, and just let go of failure. The truth is, making mistakes can actually build character. Getting busted for being a gossip, for example, can teach you that bad-mouthing people usually backfires. Keep your lips zipped from that point on, and you'll be known as the girl who only has good things to say about others.

#14 Take care of yourself.

It might seem impossible to forget to do basic things like eat and sleep but, trust us, it is incredibly easy. You have to stay healthy, so eat right and get plenty of sleep. Susanna recounts a night she chose studying over sleep: "I tried to study from 12 a.m to 2 a.m. Needless to say, I bombed the test. I probably would've done better if I had gotten a full night's sleep and woken up early to study." At times, you might have to sacrifice an A+ for some Z's.

#15 College? Forget about it for now!

Freshman year is not the time to plan your college career. You might swear, "Harvard is the only place I'd ever want to go." Kudos to you for being so clear about your goals. But chances are your goals will change. When I was a freshman, I didn't even know mg college existed. Leave the college anxiety to the upperclassmen--you'll have plenty of time to deal with that later.

#16 Life isn't defined by milestones.

You'll probably experience a lot of firsts in high school--first boyfriend, first D, first prom. But high school is more than a series of firsts and lasts--it's about all that great stuff that happens in between.

As Melissa says, "I'll always remember random times during play rehearsals when we'd all hang out and talk while we made props. Or Friday nights spent driving around, singing along to bad '80s songs with my friends." As cheesy as it sounds, the next four years are going to fly by. So make every little moment--the good, the bad and the ugly--count just as much as the big ones. Got it?