You could also think about revues or song cycles--something like Jason Robert Brown's Songs for a New World is great for a small space. Since the show is composed of related stand-alone songs without a strict plot, a small space would actually add to the show, since it's a fairly personal piece. It's also great for an ensemble cast. There isn't a leading role, so each student would get equally strong material (and their individual chance to be in the spotlight!). The other great thing is that you could use as few or as many students as you need. So each song could be sung by a different student, or a smaller number of students could sing two or three songs each.