Since your son is only 6, he probably is new to the school setting. He will be expected to display many social skills, such as following directions, staying on-task, keeping his hands to himself, sharing, etc. These skills take time to learn and master, but you can also do things at home to help him acquire these skills.
Create role-playing activities for him if he does not have siblings at home with whom he can practice. Set up situations in which you pretend to be one of his classmates. Show him the appropriate way to ask for something rather than just taking it.
Start off with easy role-playing situations, such as simply giving him a toy that he has requested. Then introduce more difficult situations, like being a friend who does not want to share. Have him show you how he would react to the situation, and then teach him how you and his teachers would like him to react in the future. Then practice, practice, practice. The more practice he gets, the more comfortable he will become using these new skills.
Also, giving a negative consequence immediately after he displays an undesired behavior will help curb this behavior. Follow up by teaching your son what the appropriate behavior is. You will probably have to do this repeatedly. Practice on a daily basis so you know he understands what he is supposed to do. Be patient; he is young and is still learning.
