Why does Greg's behavior escalate or get worse?

He was still mad about last year's punishment and wanted to show is discontentment by rebelling and fighting inside the school.

Explanation:

When the students were made to sign a contact to respect one another and avoid fighting in school, Greg wanted to show discontentment about his last year's punishment to the principal by rebelling against the contact. Therefore, he got himself into fight with some students to show his discontentment to the principal. However, the principal did not say anything to him which made him even more furious and mad. Greg then started to involve into more fights and his behavior got more worse eventually in an attempt to show his discontentment to the principal.