Yes and No

Yes, players who know they are leaving can definitely stop going to class after the NCAA tournament. Yes, they lose eligibility for the next semester, but the NCAA can't go back and void any game they played in the past.

That said, a player who leaves without finishing the semester hurts the school's APR score. With a low enough APR score the schools can get penalized, which could include a postseason ban. There is no penalty for athletes who finish the semester and then go pro. So the majority of kids finish the semester before leaving. I think nowadays a lot have started finishing through online classes. Even if they are on campus, though, you're usually only talking about 4-6 weeks after the NCAA tournament.

I know during Cal's time at UK there has only been 1 player known to have dropped out of school and leave. That was in Cal's first year at UK. Early in Cal's time at UK I know of a lot of athletes that were still on campus after the NCAA tourney but most now seem to be finishing online.