Try not to judge much by your son's reply when you ask him 'why' he did something. To be frank, that question with young children is not very useful. They do not have the cognitive skills to answer in a revealing fashion. It's clear that your son requires, as all young children do, a good amount of structure and routine. When he does not have the benefit of such structure, he behaves in a more disorganized manner than he does in the face of structure. Observing a child playing alone may not be particularly instructive. Observing him in social situations, and speaking with the people (parents, teachers) who are with him in those situations, tend to be more useful ways of figuring out what is occurring. For now, be sure he receives a good amount of structure and routine, and be sure the limit setting is clear. Classroom rules should be clear and straightforward, and violations should be met with time out.