Two main things happen in chapter 13. First, Max is called alone in to see the principal. The principal has something to say to Max about his father, but Max does not want to hear it and has a kind of fit or tantrum. The principal assures Max that "nothing will happen to him" and mentions receiving a request from the parole board, which makes it seem like Max's dad is about to be released and is thinking of making Max come live with him. We never find out exactly what the news is about Max's dad, but Max feels afraid and guilty about his own reaction—he seems to have blacked out and perhaps injured the school nurse, who was trying to help him, but he can't remember hitting her.
The second event is Kevin's allergic reaction to some chop suey he had for lunch. His reaction is sudden and severe— he cannot breathe, and his face turns purple—and Max is terrified. Max runs to get the nurse, who calls an ambulance, and Kevin is taken to the hospital. When the principal tries to empathize with Max, telling him that he has had a bad day, Max angrily says that Kevin has had the bad day, since he was hospitalized for trying to eat his lunch.
What's significant in this chapter is Max's growth, both as a student (Kevin has been teaching him how to read) and as a person. Max is aware of these changes within him and is anxious to preserve them. His great fear at his conference with the principal is that he will be taken out of the regular class and put back with the "learning disabled" kids, and his remark to the principal about Kevin's day being worse shows that Max has developed some emotional intelligence as well.