We first come to truly hate Umbridge when we discover that her idea of a punishment for students sentenced to detention is a quill that carves the message on the back of the student's hand. This punishment is obviously quite unusual due to the fact that it causes permanent physical harm (if I recall correctly, it's mentioned in later books that Harry still has scars from the punishment).
Now, obviously when Umbridge became headmaster, it's a bad idea to refuse the detention punishment because she presumably has power to expel you. But prior to that, I find no evidence that she'd be able to expel a student for refusing to do what's arguably a cruel and unusual punishment (and if she could, it begs the question of just how much she could force a student to do before the student is allowed to refuse).
So how come all the students begrudgingly put up with the punishment? How come nobody simply put their foot down and said "I'm not doing that" to her? Surely there's a limit to how much she can magically force people to do (and it seems like most students accepted the punishment).