Some naysayers may crack negatively on this, but IMO it's a good move. Problem is, most HS coaches don't need a rule like this --- it's the travel team coaches and parents who over-use a lot of young arms.
http://www.nfhs.org/articles/pitching-restriction-policies-in-baseball-to-be-based-on-pitches/
http://www.nfhs.org/articles/pitching-restriction-policies-in-baseball-to-be-based-on-pitches/