Sometimes kids act out when they know a person does not like them. Preschool is for learning and adjusting to social environments. We had this issue with my daughter....she was terrible in preschool, but in kindergarten, she was an angel...what a difference the teacher made.
With that said, My son has ADHD. There is a test that the teacher and you can take...I think it is called the copeland. This helps the doctor review his home and school behaviors. I do think your son is a bit young...sounds like a 3 year old, but if he is extreme, might be good to study him for a while. Sending you some thoughts on your new journey....it is not always easy to make correct decisions...sometimes, we think with our hearts...instead of with our minds.