I can definitely understand your concerns. It’s true that kids can have some difficulty adjusting to new situations, such as preschool and the expectations that come with being a student in a group. Sometimes in these situations we can see kids get inhibited or withdrawn. Other times we can see an increase in behavioral concerns, such as difficulty listening or problems with boundaries.
In order to know the right way to address this problem, more information is needed. I would want to know more specifics about the types of situations where problems arise, the exact nature of the behavior, and how the situations are being handled. For example, if we knew that your son gets physical when someone else has a toy that he wants, it might point us in the direction of helping your son negotiate things like sharing and turn taking without using his hands.
Sometimes, inadvertently, paying more attention to a behavior actually increases it. We can’t ignore being aggressive, but we can try to pay a LOT of attention to the things we want to see (e.g., verbal praise and sometimes rewards). For example, in the above case, if you or the teacher saw your son sharing without getting physical, saying “Wow! Great job sharing. Now it’s time to wait for your turn.”
Hopefully, clarifying the problem with the school, highlighting expectations, and developing a mechanism for how your child can succeed will help. If this does continue, and if behavioral problems persist, I would consult a specialist such as a child and adolescent psychologist.