It's pretty common for boys and girls that age to go through a pushing or hitting or even biting. Not that it's ok, but you don't have to feel like it's necessarily something that means something for his future!
I don't know how much you can do at home (about the school behavior) other than discuss using words instead of hitting. At age 2.5, the reaction is going to have to be from the teacher in the moment. Punishing or giving rewards hours later will do no good (in my experience of my boys at that age.) So you can work with the teacher to make suggestions, but she's going to need to control her classroom. Keep you in the loop, yes. Expect you to 'fix it,' not gonna happen.
Think about whether anything has changed - is the teacher the same, things at home, etc. Spend some time volunteering in the classroom if you can, because watching what triggers are there is a lot more helpful than taking a guess. Ask the teacher to keep track of what happens - is it a particular time of day? Is it at transitions? Are there specific kids that he has problems with?
My husband volunteered in my son's preschool class after a new teacher came in and my son started having issues. We were shocked at the lack of control in the classroom this new teacher had (half the kids were hitting, kicking, pushing at some time during the day - it was nuts.)
Anyway, in short my advice is to gather information, look for patterns. Behavior in a 2.5 year old is their way of communicating information. That doesn't mean we have to put up with it, but it's a lot easier to manage if we listen for the issue first.
One more thought - for at home, take a look at the book 1-2-3 Magic for ideas.