It's been quite a while since I read this chapter so the specifics aren't totally clear. But my point was, I never thought he shouldn't have been punished, but that if it was that serious to give one person a week of detention when literally no other student who participated received even the slightest reprimand, is insane. My father never would have allowed something like that to happen to me in the circumstances of the food fight. I don't see Jackson as an "ass" in this situation as he fell on his sword and took the punishment for his entire group.
Actually, maybe it's not extremely evident, but Jackson has changed and matured in some ways since the beginning of the story. At the start, all he really cared about for his final year of high school was partying and goofing off with his friends. By the end, he's way more concerned about a potential future together with Luke. Way more dedicated to his swimming, taking it a lot more seriously, and though he complains internally about some of it, working to shore up his weaknesses to make him more valuable to the college team.