Crossroads.

A school on a tough site in Santa Monica has received a building which offers a sense of place and hope, partly by the courageous use of colour.

Crossroads School in Santa Monica may not be the best endowed or appointed educational establishment in the world, but it's got guts. The site of the middle and secondary school focuses on an alley which is lined on each side with a mixture of commonplace storage and industrial buildings that have been converted piecemeal to school uses. The alley is private, but in most other respects, it is just like any other in the area, even to the extent of having to accommodate car parking. Yet it acts as the school's open-air social forum: it's the 'campus quad'.

Now, the quad has at last acquired a good new building. Steven Ehrlich's Paul Cummins Library is both a landmark and a measure of quality for the whole place. Crammed tight against its neighhours, and with a long thin plan stretching east-west like theirs, the library had to conform to many of the constraints of the campus. But it pushes out a generous porch into the quad. A canopy is flanked by a student sitting area which is enclosed by a frameless glass wall: a surprisingly sophisticated constructional device in the circumstances, and one that makes the area and the reception desk behind it both prominent and welcoming. Over the smooth glass hovers an irregular box painted a striking shade of blue - or rather perhaps pale indigo: a daring move in the area, but one that certainly attracts attention without being strident. Above and behind the box hovers the main roof plane, propped on a striking yellow steel frame and jauntily sloping up sideways to allow a north-facing clerestory to run along the whole length of the building. The roof is made the more jaunty because it slopes gently from west to east as well, an inflection much more readily perceived from inside. It is unlikely to occur to either architect or users, but to a British observer of a certain age, the plane seems to be a little like an abstracted post-modern academic mortarboard worn at a cheerful angle in the sunshine.

Inside, the big library volume beckons from a wide balcony that opens over the reception area. The rest of the ground floor is given over to mysterious troglodytic class and seminar rooms, three of which serve the adjacent mathematics centre. (The tight-packed site plan makes daylight on ground floors a problem throughout the campus.) Upstairs, the main space of the library is full of light from the clerestory (which turns at each end).

The tall volume is articulated by the strong rhythm of the straddling yellow steel frame which is completely exposed. Props on the north side are inclined, following the plane of the clerestory, making the room more cheerful and tent-like than it would have been if they had been vertical, echoing the ones to the south. Beyond the north props is a strip of stacks, and a similar lower aisle containing staff rooms and lavatories runs along the south side.

Construction is almost completely industrial, the contemporary Californian vernacular: simple steel structure, blockwork, external grade plaster and so on. But the feeling of the place is far from being utilitarian. It is celebratory and happy, an atmosphere generated by geometry, occasional use of fine detailing (the frameless glass at the entrance for instance) - and, of course, by the deft colours.