Dumbledore is the single most powerful wizard know, and the most influential in magical Britian. He runs a school where he is beloved by nearly everyone. If he wanted to change things, he easily could have done more. Especially since Fudge wasn’t very powerful and had to deal with an entire bureaucracy. Direct change at the school itself would have been feasible.

And while the parents might have threatened to remove their students, they weren’t really. Where else are they going to send their kids to get educated? There are other schools but the culture difference was so stark that seems unlikely.