SCHOOL IS BACK IN SESSION.

Byline: DAVID KRONKE

>TV WRITER

"High School Musical" is easily the most successful pop-culture phenomenon of the 21st century: a cable-TV movie produced for $4 million earns profits far surpassing $600 million, is seen by 160 million people worldwide and erupts into an industry of toys, games, CDs, DVDs and touring productions.

The inevitable sequel premieres tonight on the Disney Channel, reuniting teen sweethearts Troy (Zac Efron) and Gabriella (Vanessa Hudgens). They, and the rest of their high-school pals, land jobs at the local country club, except for the pampered diva Sharpay (Ashley Tisdale) and her foppish brother Ryan (Lucas Grabeel), who are exempt because their parents own the joint.

Along with director/choreographer Kenny Ortega, the cast happily invited us on an oral history of the first film, the resulting phenomenon and the sequel.

Many of the cast members were barely known before "High School Musical."

Grabeel: I'd go to work at Blockbuster with a (Disney Channel) movie on the shelves. One guy said, "You look like the kid in 'Halloweentown High,' " and I said I was, and he said, "Whuh? You work at Blockbuster?"

Efron: (At the auditions), for all of the partner things, it was me and Vanessa that always got paired up. At first, I think we interpreted it as a negative thing.

Hudgens: We were like, "Oh, gosh, they don't care about us."

Efron: But, for some reason, we kept making it through the different levels. I don't know what it was with Vanessa, but we clicked from the very beginning.

Bill Borden (executive producer): (At dinner during the first film), we're sitting with all these guys, and I said, "Do you realize that -- your guys' lives are going to change?"

Grabeel: Everyone was telling us it would be so big, but I was so skeptical. I still knew it was a made-for-cable movie for kids -- what could come from that? I was the person most in denial of the six of us. I thought of myself as the starving actor who was supposed to be waiting tables.

Initially, "High School Musical" wasn't going to be much of a musical at all. It was Ortega who first suggested, after the film had already been cast, adding songs and the elaborate choreography that made it a hit.

Ortega: (Despite adding the intricate choreography), that didn't change the budget. They said, "You have to do it for this." We just managed to do it by the hair of our chins.

Tisdale: We had two weeks to learn the choreography, and I was wondering, "How am I gonna pull this off?"

Hudgens: It became something different from what I thought I was getting myself into. But without that, "High School Musical" wouldn't be what it is.

The film became a massive hit; its soundtrack is the first from a TV production to debut at No. 1 on the Billboard charts -- and goes on to break record after record. It inspires a world tour (including a performance by the cast -- save Efron, who was shooting "Hairspray" -- before 65,000 rabid fans in Sao Paolo, Brazil) and scads of ancillary marketing.

Monique Coleman (Taylor): I went from "I don't think I can do this" to "I don't want to do this" to "When can we do this again?" The screaming was so piercing, it was like the roar of a waterfall.

Grabeel: The touring experience was go, go, go. In South America, the paparazzi hounded us. I felt like Michael Jackson -- there was a security guard on either side of me, pushing me through the crowd.

Tisdale: People (in Brazil) were so excited, it was like the Beatles had arrived. (On "HSM" products): The Barbie doll is really cool. I never thought I'd be a Barbie doll. It's hilarious -- there's "High School Musical" underwear. That kids can wear us is kind of strange.

Ortega: I knew it would be big, but I didn't think it would be ice tours and computer games.

Grabeel: They've marketed the crap out of it -- in the best way, of course. It'll be interesting to see where the line will be drawn.

Naturally, the sequel opens with a wildly ambitious dance sequence.

Efron: "What Time Is It," the opening number, (starts) the movie off where we left the last one, which is with our biggest production number. There were so many sections to filming it. I remember, day after day, it was like we haven't finished "What Time Is It" yet. But, again, it's one of the more exciting numbers in it. We are going to start it off with a bang, you know.

At one point -- the film's final emotional moment -- the cast took over and insisted on shooting a scene beneath the pour of lawn sprinklers on a golf course.

Ortega: It was freezing cold, under 30 degrees. And as responsible adults, we couldn't subject the kids to this. We just decided, we're not going to do it.

Coleman: It was freezing, and we were to go running into the sprinklers. The executives said no, we couldn't do it, but we badgered them until they let us do it -- we said, "What if it turns out as great as we want it to be?"

Ortega: We were gathering the cameras and all of a sudden, we're told, the cast wants a private conversation. All of them. They said, "Kenny, we're doing the sprinklers. -- Damn it, you can't take this away from us." That's just another tribute to their passion and energy, which was why the original film was a success in the first place.

David Kronke, [phone]

[e-mail] www.insidesocal.com/tv

HIGH SCHOOL MUSICAL 2

>What: Sequel to the wildly popular 2006 movie.

>Where: Disney Channel.

>When: 8 tonight through Sunday.

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Photo:

Summer beckons for the smitten teens played by Vanessa Hudgens and Zac Efron in "High School Musical 2," the hotly anticipated sequel to the made-for-cable Disney juggernaut.