HS football participation is down (at least in the areas where I coach and the schools in the area.) but I don't know how much of it has to to with head injuries as much as it has to do with how freaking long football has become. It is brutally long and some coaches really have a hard time "getting" it.
Example: 9th-12th graders where I coach at are expected to meet 4 days a week in the summer. 1 hour of on the field conditioning and plays...1 hour in the weight room.
That is all summer with the exception of two days off for the 4th and one week off before real practice starts. Then real practice is 5 days a week with games on Friday, so a 6 day commitment. Season ends...two weeks off (sort of) and back in the weight room 4 days a week and more conditioning.
It is brutal.
We've done this to almost all sports. Hockey, volleyball, softball, baseball, basketball even dance. And it's sad, but that is what it takes for those who want to compete at the highest levels. When I was a kid and into high school, a person could participate in multiple sports. i.e. Football, basketball or wrestling, track then baseball. I don't see very many kids doing that anymore. If they don't focus and specialize on one sport they won't make their HS varsity team or the club team they want. I'm not sure exactly what has caused this. My guess is its the culmination of parents living vicariously through their children. Both of my kids could've continued with 2 or 3 sports but they were forced through circumstance to really focus on only one and treat another as a sideline. Granted neither had/has D1 talent but it's sad that they could not participate in all the sports they would've liked to. If you don't commit to a sport year round, chances are you won't be playing that sport for very long.