For me, I just teach them the lift and have them slowly work up over several workouts. Then I just watch bar speed and go from there. As long as you have a starting point, you will be good to go.
After that, design your workout. The key things are this:
Eliminate any distraction/confusion - this means you carve anything confusing from your program. Large groups of kids will usually screw things up, especially in the beginning. So keep it as basic as possible UNTIL they get the hang of things. For example, all we did in the beginning was DB squat supersetted with push-ups - all done as a group. Slowly add in things IF they add value. If not, don't do them.
Be organized and spell everything out. Don't think they know what "warm-up" means.
Don't give them any choices in the beginning - tell them exactly what to do and when to do it. As they get more experienced, you can give them some room to make choices.
Keep things as basic as you can and err on the side of "too light" until they are comfortable with the movements. The off-season starts next week for us and the training we will begin with is very, very simple. Remember that sport training is more than just lifting, so be sure you are training all aspects.