Why could students not simply say "no" to Umbridge's detention punishment?

Question

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).

Answer 1

Actually, apart from Harry, no one else seemed to be getting detention from Umbridge (at least until she became Headmistress). As mentioned by @rand al'thor, Harry refused to complain about her even though he was urged to do so by Ron, seen in Harry Potter and the Order of the Phoenix, Chapter 13: Detention with Dolores

“Yeah, so do — Harry, what’s that on the back of your hand?” Harry, who had just scratched his nose with his free right hand, tried to hide it, but had as much success as Ron with his Cleansweep.

“It’s just a cut — it’s nothing — it’s —” But Ron had grabbed Harry’s fore arm and pulled the back of Harry’s hand up level with his eyes. There was a pause, during which he stared at the words carved into the skin, then he released Harry, looking sick.

“I thought you said she was giving you lines?”

Harry hesitated, but after all, Ron had been honest with him, so he told Ron the truth about the hours he had been spending in Umbridge’s office.

“The old hag!” Ron said in a revolted whisper as they came to a halt in front of the Fat Lady, who was dozing peacefully with her head against her frame. “She’s sick! Go to McGonagall, say something!”

“No,” said Harry at once. “I’m not giving her the satisfaction of knowing she’s got to me.”

“Got to you? You can’t let her get away with this!”

“I don’t know how much power McGonagall’s got over her,” said Harry.

“Dumbledore, then, tell Dumbledore!”

“No,” said Harry flatly.

“Why not?”

“He’s got enough on his mind,” said Harry, but that was not the true reason. He was not going to go to Dumbledore for help when Dumbledore had not spoken to him once since last June.

These are the reasons Harry didn't mention it to anyone. The only time the book mentions anyone else getting this sort of similar detention is Lee Jordan, and by then Umbridge (though not Headmistress yet), could only be superceded by Dumbledore and even upon his intervention might have just brought in another Educational Decree that stated she was in complete control of all detentions.

To be fair, once Dumbledore was gone and she became Headmistress, the students were much more rebellious and she seemed to have less control than ever.

Answer 2

In addition to what ʀ _ ɪʟ226 said, my experience tells me that even in unjust situations people can be very hesitant to do something against the person causing it because of fear.

In my sixth degree at school I had a teacher who used to be quite authoritarian against us, and we also had far too much homework. Even if I tried to convince my classmates to do something against that situation, they were too afraid to be "punished" if we all together spoke up! That the situation was punishment itself didn't mater.