I have to admit I've gotten swept up in the talk about hard throwers at times. But we forget that there are scores of pitchers who succeed, particularly in high school, without throwing hard, or even because they throw SLOWER than most.
A high school kid who throws mid 80's is probably going to be able to get it by a lot of hitters, but not all of them. Upper 80's, most of them, still not all. 90's, probably all of them.
A kid who throws low/mid 70's can also be successful. This is the guy you never want to face because he messes up your timing so bad, you don't hit for the next three games. You go up there dreaming of hitting it to the moon and you pop out or dribble something that barely reaches an infielder.
If you "average" the heat guy with the finesse guy, you get low 80's. So I guess you could say the "average" high school pitcher throws in the low 80's. But if I had three guys to choose from, "average" is the last guy I'd send out there. These guys tend to look a lot like batting practice, unless they've got something else going for them like a nasty curve, sinker, spot control, etc. The high schooler who relies mostly on 80-82 fastballs gets tatooed.