Professor Umbridge knows that she won't be able to command authority with her students through any of the usual means—by earning their respect or admiration. So she's happy to do it through a naked power grab.

Umbridge uses her connections at the Ministry of Magic to have Educational Decrees proclaimed. These decrees are nailed to the wall outside the Great Hall and specify how things should be run at Hogwarts. Students aren't allowed to form groups. They are forced to submit to questioning. They even get to sign up for her Inquisitorial Squad for extra credit. Ugh.

Of course, Dumbledore also finds a way to work around the decrees when Umbridge fires Professor Trelawney:

DOLORES UMBRIDGE: Dumbledore, may I remind you that under the terms of Educational Decree Number 23, as enacted by the Minister—

DUMBLEDORE: You have the right to dismiss my teachers. You do not, however, have the authority to banish them from the grounds. That power remains with the headmaster.

DOLORES UMBRIDGE: For now.

Hey, he's technically right.

The Educational Decrees are symbols of Umbridge's power within the halls of Hogwarts. As more and more of them are nailed to the wall, her control of Hogwarts grows and grows. That's why it's oh so sweet to see them come crashing down around her when Fred and George fly through the Great Hall with their firework display.

Justice at last.