I taught middle school students for 39 years. I loved it, and I loved the students. But I am having a real problem thinking of items that might sell at a craft show for that age. This sounds a bit awful, but students that age are pretty self-centered, so you might try for some items that they would like for themselves, as I think that is what they might buy. Scrunchies for the hair, magnet things for refrigerators that they might want inside their lockers, fabric bracelets (? I think I saw such a thing somewhere....)...
Anyway, I am really not much help, am I? Students will also be looking for items to give to their friends, maybe?
Dina