Have you considered doing a cutting of a full-length play? The ones that I've seen that have really knocked my socks off have been great trimmings from longer pieces.

Aw shucks.

David Lindsay-Abaire, for my money, is the funniest author out there today. (You'd probably have to tone down some stuff for school, but a play like Fuddy Meers might be possible.)

We're actually in rehearsals now for a set of one-acts for June.  We're doing three short pieces by a guy named Walter Wykes:

The Tragical Tale of Melissa McHiney McNormous McWhale: it's a very short (6m) piece for a mixed cast of 6-12 that's essentially a poem narrated by a Greek Chorus of Vegas-type characters.  Fun with the sets, costumes, etc., but not much on the acting side of things.
Family 2.0: Essentially a longish sketch (10m) for a cast of 5 (3m, 2f).  It's a pretty standard comedy that's a little risque.
The Spotted Man: This one reminded me a lot of Durang.  It's medium-length for a one act (~22m) and is essentially an actor piece for the m and f parts (the f part is particularly challenging, since it's actually 3-in-1).  There's also a scene-stealing bit part with no real lines for another m.  Has an absurdist, kind of shocking ending.

We're also doing "Hidden in this Picture" by Aaron Sorkin (4m, ~40m) which was later turned into the second half of a full play, but word has it that "Hidden" is the only half worth doing.  It can be done on a bare stage, and is a very wordy, dialog-driven comedy, as you'd expect from Sorkin.  I'm incredibly fond of this one.

Students love 'The Bear' by Chekhov.