For those who have son's throwing hard in HS, and not using a change, go back to justbb's post.
His son has the most wins in the Padre's Milb organization each of the past two years.
Coming out of HS, he was widely considered one the best HS pitchers in the nation, and would like have been a very high round pick had he not chosen Stanford. We have season tix at Stanford just behind home plate. I watched justbb's son his senior year as he won 7-8 straight complete games. His change was unhittable, thrown in any count any time in the game because it looked just like his fastball...until it wasn't.
Having command of the change to be able to throw it in any count, in any situation, is a huge advantage.
Being able to throw it with the same arm speed and slot as the fastball is equally important.
I am coming at this from the perspective of a parent of a pretty good Milb hitter. For him, fastballs up to 100mph could be turned around.
Good curveball was trouble but with adjustments, could be managed, especially after he had seen it a few times.
A 2 pitch pitcher was in big trouble the 2nd time through unless they had Randy Johnson type stuff.
A pitcher with 88-92mph, with a change at 78-80, thrown in any count with command, which looked just like the fastball coming out of the hand, was darn near impossible.
When looked at in the context of college baseball, 85-88 is fine, and get to 90 and you overpower HS hitters. College hitters are NOT HS hitters. They get used to 90 in a hurry.
In the experience of our son, the higher the level of baseball highlights the value of the change up when controlled with command and the ability/confidence to throw it any time.