I just directed a play at school, (went very well, everyone thought it was pretty damn good, even the usually critical people, anyway) and I did Hayfever by Noel Coward.
Cast of 5 boys and 6 girls. It's about 90minutes long and pretty easy to stage if you've got some furniture. I'd recommend that or maybe a Pinter if you like his stuff, can't say I do though.