If by "average" you mean the fast ball velocity of the 5th out of 9 pitchers on the typical high school team, that's about right.
If by "average" you mean the typical pitch thrown in a HS game, that's wishful thinking for the velocity-challenged. That's because maybe half or more of the innings are usually thrown by the top 2 guys on the team, and then maybe 2 more guys throw another 35-40%, and the rest of the staff gets an inning here or there. The guy who throws under 80 may never pitch at all.
In our district (7 teams, AAA), we have one kid who throws 90-92 and four who are in the 85-89 range. There are any number of kids in the 81-84 range. At the low end of that range you will get pummeled unless you have Maddux-type control, movement or off-speed stuff. (Or maybe just a classic "crafty southpaw".) In high school, it's easier to find hard throwers than Maddux types. Lots of big strong kids, but it would be expecting a lot to demand that a teenager truly master the art.
If a kid is throwing 78-80 as a freshman, he can look forward to maybe building to 85-86 by his senior year, which is good enough to be # 1 or at least # 2 on most teams. But you may have to wait your turn and take your lumps on the way up.
You'd still better learn how to PITCH. Even the guy who throws 90-92 has gotten burned some when his location is off, when he hangs a curve, or when he pitches behind and has to come into the hitting zone over and over.
I can't speak for other parts of the country or for smaller (A or AA) school districts, but I can say that the above also tracks what we see in travel ball.