Easy Lunches

I am in desperate need of some brilliant lunch ideas. I am a teacher, which right now means I am teaching summer school. We don't have a lot of time to eat (15 minutes), so I eat with my students and I eat fast (which I know isn't ideal). I had been taking a turkey wrap, but they aren't as low calorie as I'd like. I am very active at summer school (we have a lot of outside time), so I definitely need something that will last me the whole afternoon ... thoughts?