There was a time when the San Francisco Circus Center would have been called a "clown college" and would have devoted itself to turning out white-faced jesters bound for three-ring extravaganzas. But that was before the Great Circus Renaissance set into motion by Cirque du Soleil. These days the SFCC offers adult and children's classes in such big-top diversions as stilt-walking, trampoline, tumbling, and aerial arts. The school also runs the intensive yearlong Clown Conservatory, whose Web site is awfully serious for a training program intended for pie-throwers; it's full of high-minded rhetoric about how clowns can "make deep contributions to our communities." We assume that's a riff on the "so many clowns in a tiny car" gag -- which issome contribution to our community. Be that as it may, the Clown Conservatory concentrates on the clownish arts of juggling, acrobatics, and mime, as well as on clown history, makeup application, and the finer points of terrifying little children.