The biggest single culprit for fb injuries, including head injuries/helmet related (spearing, which is still done although less often) is the first down. O players give it everything they've got to make it and D players give everything to stop them. In so doing, players expose themselves to injury and injure the other guy.
Remove the first down altogether. Give each team X number of plays on offense and you either score or turn the ball over on downs (trying for the score on the Xth down) or punt. Granted, there will still be that all out try on the Xth down, but this will be much less frequent, most likely near the goal line.
I realize that this goes against our way of thinking. We are a make progress-consolidate-make more progress culture, but it has created a dilemma as kids get faster and stronger, while we encourage them to maximize their talent and incentivize them big time to do it.
There are other things that would reduce injuries, such as natural grass and removing protective equipment (counter-intuitive, I know, until you look at rugby), but this would be the easiest thing to try. You could do this for 5 years, say, and reevaluate then. Or, do it in Divs 2 and 3 and see what happens.