I suspect that this was the final straw in an entire heap of incidents between the bully and Anna, and the bully and Todd, outside the notice of adults - in hallways, on playgrounds, in cafeterias. I also wouldn't be surprised if Todd told the bully beforehand if he said another word to Anna, he'd punch him in the face, and the bully decided to be an asshole in the classroom ON PURPOSE, thinking that Todd wouldn't dare to hit him in front of a teacher.

Take Todd out for pizza and teach him that the only people you should punch are Nazis.