Why does Jack refuse to tell both Mr. Tushman and his mother his reason for punching Julian?

Jack refused to tell his mother and Mr. Tushman what actually happened because he didn't want to make things worse.

I shrugged but I didn't say anything. I just couldn't. If I told him that Julian had called
August a freak, then he'd go talk to Julian about it, then Julian would tell him how I had badmouthed August, too, and everybody would find out about it.