Preschool Behavior Trouble

So my son started preschool (first school/daycare ever) three weeks ago and he only goes two days a week. He has a tendency to hit when he gets mad or doesn’t want to do something and the first week he was an angel but the second week he hit his teacher when she was trying to make him pick up a toy he knocked down. Sometimes when he didn’t want to change activities he would get stubborn and upset and try to hit or bite. Obviously this is not okay when he does it at home and obviously we’re mortified it’s happening at school. We’re definitely working with him and instilling “hitting is not okay” we’ve read books with him, etc. The teachers and director invited me to come observe him at school and today went great. Only one incident where he tried to hit, but he quickly calmed down. I know he didn’t learn this behavior (he started at 2). And I hope it’s a phase that passes very soon, but I feel like his teacher acts like he should not be doing this at this age. He’s also one of the youngest in his class and has always been late on his milestones. She asked if we let him hit at home and if we’ve ever consulted professional help. My first Mama Bear instinct was to feel insulted. And this can’t possibly be the first time she’s dealt with this in her 30 years of experience?!? She also asked what should she do when he lashes out because she’s got 20 other kids to keep track of too. Isn’t this part of her job? I’m doing my part at home. This is part of why we put him in school. To learn social skills. He’s also an only child. Am I wrong to feel confused here?