First of all, don't get too worked up about this. I got a call on the first day of Kindergarten (five years ago) and thought "OH NO!" He's going to be labeled a troublemaker, disrespectful, difficult, whatever...which panickec me because I knew he wasn't any of those things. And after that little bump in the road the first couple of weeks of school, we've never had a problem.
Anyway, I went in for a quick pow wow with the teacher before school my son. She and I let him know that she was the absolute boss in school and he would obey her. No asking, no cajoling, no question...just reiterating that she was in charge. Up until then, she was basically nothing to him. Perhaps a babysitter type in his mind whose role was to just keep him safe or something? When he got that she was standing in my stead at school, he got right in line. And she also had to take a little time to get to know his personality and how to approach him, which she did invest the time to do.
She also came up with a little system whereby she would put a piece of masking tape with a smiley face drawn on it on his shirt when he left for the day if he'd done what he wa supposed to, and we'd have a tiny reward for every day that happened. I never got another call again.
The main thing is, though, don't get too upset or read too much into it. He will like school more if he's not in trouble, and the only way he's not going to get in trouble is to follow the rules and instructions, so just remind him how important that is. He'll get it, and as his teacher gets to know him, she'll find ways to gert him in line.