Answer:
The students probably feel helpless and betrayed by the administration that previously promised to protect them. If they all signed the same document stating the parents will be called they expected measurements to be taken if they are fought with.
We can read that those injuries that students get in the fight with Greg are not that mil or harmless, yet the school administration does nothing. Students therefore probably feel threatened, helpless and made to fight with Greg in order to protect themselves, as the school is not willing to do that for them.