can I suggest you try www.plays4theatre.com - the home of New Theatre Publications and The Playwrights' Co-operative. There are several plays on there that might meet your needs, go to the catalogue page and check the "Youth" button to get a list. It's particularly worth look at the works of David Walker, especially "Acting Young". If you find anything you can have a copy sent to you (for evaluation purposes only) free of charge.
Best of luck