I coach JV at a 3-A high school (1200 kids). Over the past few years one thing I have enjoyed doing is after my JV game is concluded I go up to the press box and watch the varsity game. I often have my radar gun with me and discreetly take readings during the game.
I can confidently say the average high school pitcher that I see tops out somewhere between 72 and 78. Sophomore's playing varsity tend to be the 72 to 74 range and juniors and seniors anywhere from 75 to 78. It is a very rare occasion that I see any velocity 80 or higher.
But once in a while I will see a kid hitting 85. And I can guarantee you that is a kid who puts a lot of time in, studies the game, and works very hard to hit that number. Most of our kids where I teach and coach do not play year around. They are 3 to 6 months players at most. That being HS season, then Legion ball then put the glove away at the end of July and not take it out again until high school tryouts next spring.