I suspect most students have done at least some level of test prep for a long time, probably going back to early versions of college admissions tests in the 1920s or before.
FWIW, when I was in (good public) HS in the mid-1980s, normal (for the smart kids) was some exposure to verbal-SAT-type questions in English classes (analogies and antonyms, IIRC), plus the PSAT (nominally a practice for the SAT) in 11th, plus a ~dry run on the SAT late in 11th, and the big try early in 12th. Plus all the standardized testing we’d taken from about 3rd grade on. Plus, test prep books if not classes existed and were used, too.
While it’s likely that test prep has intensified in some corners in the modern age, it’s not exactly like there was a golden age where elite students walked into a classroom with a “what, SAT? OK, I guess I’ll take this” attitude. (Notwithstanding the occasional humblebrag by folks who claim to have been hungover on test-morning due to partying the night before…)