Hi there. Sorry you are having these issues with your son. That's never fun for a mom to deal with!
So, a couple of things come to mind. First, often kids ARE better controlled with family and at home because . . . without our knowing it, they are actually exerting a lot of control. They are our kids so we let them win at games, we let them choose what to play, we give them options, etc. Others that live with them such as siblings or cousins they play with often are used to them, so they tend to kind of go with whatever the dominating cousin or sibling says. I have two boys--- my oldest had issues similar to your son minus the physical outburst. His little brother, just 15 months younger, was his side kick. My son tended to drive the way things went. At home, he was comfy and he didn't appear demanding, etc. But once at school . . . where he had NO control, it was hard for him.
Now my son has sensory issues and was diagnosed officially at 4 years old (first evaluated at 3.5 and it was inconclusive). He has spent a long while seeing an occupational therapist to work on some of these skills= managing his emotions, maintaining focus, peer interactions, etc.
If our son is standing out, I'd do two things. First, close down the behavior with limits. The school should A. give him choices whenever possible (do you want to walk by Billy nicely without touching him or do you want to hold the teachers hand down the hall?). This will make him feel like he has a little control but the teacher limits what he choices are so she ultimately has the control. B. have FIRM boundaries with consequences at school. If he hits, X happens (and X is NOT good). But this has to happen at school.
At home, read books about emotions and talk to him about it. He's feeling something before he hits someone, what does he feel? Help give him the language to discuss it using his words. Lots of books written for children to help with this. Also, there is a series of books that has "Hands are not for Hitting" which I highly recommend. At home, he is no longer king. (I know you didn't treat him as king as I didn't my son but I had to reel things back and strip back my letting him dictate too much even in his sweet way). I had to choose what we played sometimes and made sure it was different than what he wanted so he got used to not being in charge, he had to lose at things so he could experience that, he had to share things he didn't want to do, he had to do things he didn't want to do, etc.
If you see no improvement, I'd look up sensory integration disorder and see if anything rings any bells for you.
I have some in class things that can help== what do they do now with the bad behavior?
All of Specialmoms ideas are excellent and worth paying attention to.
I do have a few questions. You said he is 4 - When is his birthday?
You said he is starting school. What kind of school? He is too young for Kindergarten. Is this a public preschool or a private one?
Specialmoms comments about how the school can work with him is important. Their first action should not be to call you and tell you to pick him up. If you have not already done so - I think it is very important to visit the school and see what is going on.
There are things that you can do at home to help him at school. The book Specialmom recommended is a good one. Also take a look at "Know and follow Rules," and others listed below in this link - http://www.amazon.com/Follow-Rules-Cheri-Meiners-M-Ed/dp/1575421305/ref=pd_sim_14_5?ie=UTF8&dpID=61HJUw9wxKL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=0P352PVS7QSJ6VR7HNN7
Basically, I am trying to figure out how much of this is due to the school possibly not dealing with him as well as they could - or how much might be due to him. Oh, and how long is he at school? By the way, age is important! How does he compare to the others in his class age wise?
It could possibly be because doesn't see why he needs to obey his teachers and is so used to winning at games at home that he assumes he should at school, too. It could also be because of a disorder, but I have little knowledge about disorders despite having a couple. Oof. Usually when kids I'm around get physical, I just stop the fight and then a higher authority takes care of it. Sorry I'm unable to help.