At this age, your kiddo is likely to display anger. Your little one has no self-regulation at the moment hence he is prone to lashing out when he is angry.
What you need to know:
The goal is for your child to use anger as an impetus to change things as necessary so the situation won't be repeated. This may include moving his treasures out of his little brother's reach, or getting parental help to deal with a bully. It may also include acknowledging his own contribution to the problem, so that he resolves to do a better job following his parental rules, or comes to class more prepared. Your child may learn to see the other person's side of the issue and look for a win/win solutions to the problem, rather than just assuming that he is right and the other person is wrong.