You aren't alone. my 28 month old was out with his dad at the playground and completely went over, shoved, hit, then kicked a little boy (who was much bigger than him) for absolutely no reason other than maybe what he thought was his territory.
I was stunned. I couldn't believe what I was being told. My son is so sweeet and innocent.. so i thought.
After much research, because I was highly upset, turns out it's normal. Your son is much older, however, perhaps it is a phase.
Also, I spoke with my son about pushing his sister the other night. I was telling him no, not with abby.. no push.. and he looked at me and to my big surprise said, "I like push" - as in he likes it when I rough house with him and push him, and that is when I learned he associated all things he likes with all things someone else will like.
He might not be doing things becuase he is angry but simply because he wants to play.
I didn't realize how much my hussband and I rough house either until we tried to figure all the rough housing he was doing.
It's a battle either way, and maybe you could go and sit in the classroom unnoticed to see for yourself what the teacher is talking about.
I wish you the best, you aren't alone.
J.
p.s. Your son may not understand his new routine being so young. He may not associate his punishment with something he did so much earlier in the day. I would check into devlepment on that. Also, fisher-price.com and pampers.com both have development articles you can research. Check out those sites too. :O)