A HS kid shouldn't throw over 100 pitches
Sorry, but that is a ridiculous blanket statement, as is the statement that 4 days rest after 8 innings pitching is "child abuse*
Depends on the pitcher, depends on the weather, depends on the number AND TYPE of pitches thrown. some kids should be done at 60 pitches, some can throw 120 and be fine (assuming looking at the parameters above).
Had a kid a couple of years ago go 8 innings, threw 96 pitches on a Monday. He was good to go on Friday, ended up going 6 innings and throwing 65 pitches. His mechanics were such that we never worried about the stress on his arm (nor did his parents or his summer coaches). You have to learn each individual kid.