Having a middle school study hall makes me think back to my days in adolescent hormonal limbo. During study hall, I signaled across the wide open cafeteria (yes, study hall was in the cafeteria; one student sitting every 20 feet or so) and sold AirHeads out of a wadded up brown lunch sack for 25 cents each. Quite a profit if you do the math. I mean a box or pack must've cost no more than a couple bucks. I don't remember how many AirHeads were in a pack but I know we (me and my fellow pimple-faced entrepreneurs) wouldn't have sold them if it weren't turning a big profit.
Teaching, or simply being around youngsters, gives you so much material for middle grade/YA characters. If you don't have direct connections with kids this age, then reflect on your own adolescent vaccuum from 12 - 15 years-old.
What sly things did you do in middle school?