How can I help him develop self-discipline during school hours? It's hard to imagine this behavior from him because he's not a difficult child at home. Is it because I communicate differently and have more patience? I've observed him with his friends, and I still don't see this side of him.
Stress with him that although you do not see him behave this way, you do acknowledge that the teachers are seeing the behavior. Help him recognize his different behaviors and ask him to make a list of situations in which he behaves well and a similar list of situations where he does not show the same control. Assist him to identify what elements "trigger" his behavior. Once he can tune in to the environment enough to identify what sets him off, he can begin to take steps to regain control. Ask his teachers to assist in the analysis, so they will know he seriously wants to change. They may be able to make minor changes such as a seating change to help. Try to adapt some of your home strategies to resemble more the school environment so that your son will not see his worlds as so different.
Ask the school counselor for information about group programs to help him deal with hurt or angry feelings in more constructive ways. If the school does not have a group program, the counselor may be able to direct you to local community options.
Encourage your son to participate in a non-school group activity, such as a service project or service club. By helping others, he will feel better about himself. Also, in a new environment, he might be on his best behavior. His new patterns of response could enable him to break the bad habits of response he has developed.
Self-confidence comes with success. So long as his behaviors bring negative attention, it will be difficult for him to feel successful and confident. Change is necessary for him to break away from unsuccessful behaviors. You can be a wonderful support if you remain consistent and constant in your attempts to help him resolve this conflict.