how is his diet and how much electronic time does the get?
I know that seems simplistic but I really really do think that some kids are extra sensitive to sugar and dyes and the hynotic screen time. Those are changes you can make a home.
Also, make sure he isn't picking up any stress from you at drop off, It doesn't sound like you are lingering or anything , but a quick cheery I love you, I know you are going to have a great day see you at 3 pm And Leave is the best way to quickly get him to separate.
Maybe when you talk to the teacher, you could tell her that you would be willing ot have him paired with another little guy or gal that has a calmer personality and would be able to be a settling influence on him, you know how some combos just winde each other up? if she can steer him to a clamer kid and then you can have playdates outside of school that might help.
Teach him deep breathing, Yoga is great for increasing kids concentration and ability to be still, I've used the kids yoga dvd's from our library with my 2 and they like it.
Try not to worry, the teacher really does want your kid to succeed and if you are wiling to be open and communicate with her it will go a long way.