Carnegie Mellon’s website used to say that Subject Tests should be taken junior or senior year. I emailed them to check, because my son took Math II and Physics in June of his freshman year after taking AP Physics B and Precalculus. They emailed back, “We will accept standardized test scores from your sophomore/freshman year; you do not need to worry. Your son’s current plans for taking tests will work fine for our admissions process. We just prefer scores from junior or senior years.”
I checked and their website now says this regarding SAT/ACT and SAT Subject Tests, “Carnegie Mellon would prefer that applicants take these required tests in their junior or senior year. Test results from the junior and senior year are the best indicators of a student’s current ability and potential for immediate success at Carnegie Mellon.”
I’ve considered having him take more than those two. He could probably have taken the Chemistry subject test last year after AP Chem. This year, I suppose he could consider taking US history or biology after the APs in those. I’ve looked briefly at some sample Literature questions, and he could probably do well at that one now. I don’t suppose there’s much point to all that, though. He won’t take any more unless he takes the practice test and gets a high score on that.
I’m pretty sure the regular non-AP classes at our HS would not work at all as prep for the SAT Subject Tests. They are not required as prerequisites for the AP science classes. There aren’t any honors classes between regular and AP.