What exactly are your time constraints? What I mean is that most high schools end at 2:30 or 3:00 and then the kids have nothing afterwards. Also, most schools have a two bus system, one bus at like 4:30, then a 6:00 bus, so rides aren't an issue. Does your school not offer a system like this? I guess it could be a weightroom issue which would limit your access, but if not . . .
I think some easy to teach lifts that have tremendous benefits are the one-arm DB snatch and the heavy medball toss. Couple those with some squats and you will see some results.