I must say that not all schools/teachers reserve sending kids to the principal as a last resort for extreme behavior, some are VERY quick to do it, especially I think when the teacher is ill-equipped to handle the issue, maybe jut due to lack of time and staying on schedule. It would be helpful to learn from the teacher, and/or the principal, what the standards of discipline are -- what's the first thing they do, how did it progress to going to the principal?
Also, I agree with the statement that he's still learning how to behave in school. That's much of what kindergarten is, and I would wager that he'll get it eventually and the principal visits will taper off. What can you do to help, aside from the great idea of playing school at home and reinforcing the rules... positive reinforcement. Stop concentrating on punishing the bad behavior with time outs or toys taken away. Instead, set up a reward system for good behavior. Get a ticket for every good day at school, 5 tickets means he gets to choose a video to rent or something, some special thing that he will want to work towards. Rules and groups can be hard for independent kids, and he's been allowed to do his own thing for so long, that the adjustment must be hard for him. Recognize that it's going to take some time, be gentle but firm with him, and remind of the rules (listen to your teachers, keep your hands to yourself, wait your turn... use positive phrases that are easy to remember) and he'll get it. It's going to be OK!