I had terrible form when I first started running, then I started running more and more and guess what--my form get better as I ran more! Their bodies are not going to shatter from running a little with a funny stride. 40-50 miles a week for a decent high school boy is going to take them about 50 minutes a day of running.
50 minutes is not a lot of time. It's about enough time to prepare them for the harder training of college, teach them good form, and give them a taste of intervals. I'd say that 95% of how I run came from just naturally running that way for long periods of time. The more I ran, the better my form got. The other 5% does come from things like coach observation, drills, strengthening a weak muscle group, etc.
There's been so much focus on form in this thread, I think it's silly. If you're running 70+ miles a week, chances are your form is going to be pretty much optimal. Everyone has that one "pet issue" they have to think of- maybe some people cross their arms when they're tired, or others lean back too much, or whatever. Point is, practice the pet issue in interval workouts when youre tired, but otherwise, jeez, just let the body run. If you're doing the right stuff- practicing form when tired, hill sprints, running a good amount of volume per week, etc, then you don't have much to worry about.