Our 2021 goes to a very high SES private high school. We have seen it all with test prep - summer programs, before and after school classes, private tutors, obsessed parents, etc.
In the end, so far the kids are scoring exactly as my own kid would have predicted back in early freshman year. The high ability kids were instantly identifiable, as were the lower ones. Their scores two years later reflect that. I honestly don’t think test prep has moved the needle very far, at least as far as the PSAT/NMSQT and SAT/ACT results so far. It is a fairly small school, my own kid helps many others with informal tutoring in math and grammar (writing), and we are connected with many parents, so I feel my opinion is fairly well informed.