I had a show with a group of 8 kids ages 7-12, who were only needed occasionally, but had to sit quietly in a relatively small room while we rehearsed other scenes.
While my group was a bit younger than yours, most of them brought in gameboys (I don’t know if they’re still called that, but they’re the hand held video game things

, and as long as they muted the sound effects on them or wore headphones, they could be happily and quietly absorbed.
I second nmno’s suggestion of getting the parents involved. Let them know the situation, so they realize it’s OK for the kids to bring in ipods, books, magazines, games, laptops, anything quiet.
Make sure you have a frank, unpatronizing discussion with the teens as well; tell them that you know it’s boring, but waiting and watching are as much a part of theater as actually being onstage. Tell them that you really appreciate their patience, and know the set-up of the hall makes it even harder to stay quiet. Spell out the fact that noises are distracting to the people trying to rehearse on stage, so keeping their voices down is the considerate thing to do for their fellow performers. Ask them for their own ideas on what they can do quietly, what the challenges are to staying quiet, and how they can help each other overcome them.
If you have an ASM (or can recruit a rotation of parents, since it’s community theater), maybe you can have someone act as kid wrangler, so at least you don’t have to personally walk backstage every time the noise level goes up. Any chance they could go into a different room for a while, if you had an extra person?
Good luck!