This is probably selectively biased toward higher speeds (because generally only better players attend and get measured by PG), but the player profile pages on Perfect Game report where a kid's FB velo stands relative to his class. According to PG, the average velo for the Class of 2014 is 80 mph; for the Class of 2015 it is 79; and for the Class of 2016 it is 76. I suspect if the pool of players being "averaged" was all HS pitchers instead of the subset of HS pitchers who attend and pitch at a PG event, those numbers would be a tick or two lower across the board.