You know, that actually happened. With my eldest. After a year of what I came to recognize as intolerable treatment, I came to his courseroom, took him out of class, told him to clean out his locker, he wasn't coming back anymore. To the counselor who said, "You can't do that" I replied, "Watch me." I was scared to death. I'd never done anything that bold. Turns out, it was one of my best moments as a parent.
I just met a woman recently who did the same thing. Her son was being bullied, and the school was more concerned about him "hitting back". So, she just walked in one day and took him out of the school. She didn't strike me as the type who would challenge authority like that, either, and I'm betting she probably never did anything like that in her life. But, now, she has started homeschooling him.