Disclaimer: I do not own Harry Potter. Else Umbridge will suffer a worse fate than centaurs.
Take Zero
It was not a secret that everyone in the castle hated Umbridge and wanted her gone. Well, everyone except Argus Filch, that was, but he did not count.
Of course, the majority of them being teenagers, they were used to getting what they want. At first, it was glares and sneers in class. They had hoped that negative pressure would subtly take its toll on her.
Umbridge was completely unaffected. In fact, she seemed to be in her own world every single time she gave a speech proclaiming the Ministry's greatness and how school children – like the little sweethearts in Hogwarts – needed to learn their theory in a nice and safe environment.
That was proven once, when during a speech, she did not even notice Nearly Headless Nick walk through her.
So they resorted to public humiliation to drive home their point. Subtlety was completely lost. In fact, it had been painted in large neon letters on a banner that proclaimed "Get lost, Umbridge!" A pity that Filch took it down before she saw it.
Umbridge still did not appear to get it.
The most successful pranks, of course, had been dutifully documented in history. Everyone knew of the two major pranks by Fred and George – the fireworks and the portable swamp. However, there were many, many more that were not.
This is war.