I was in school in Wisconsin when they got rid of the teachers union; schooling improved drastically. We had a physics teacher who went years refusing to write AP level physics courses when it was a union job, but suddenly found the motivation once the union left. Most substantially, at the state level the governor passed course options, which allowed high school students to take courses at local universities or other schools if their school didn't offer higher level courses.

Obviously unions aren't designed to protect students, they represent workers, however their negative impact on the quality of schooling students get is often quite significant despite being overlooked.