I use time outs, but only when I catch the child in the act and she knows she is doing something that is against the rules. Will your child understand why they are being punished if they are not quite at the age of reasoning?

Whatever punishment you decide on, you might try role playing to help the child see where their behavior crossed the line and how they might react better in the future.

As others have said, I think less about punishment than about how to make it a learning experience, and try to make sure it doesn't happen again. We obviously don't know the details of what occurred, but you might even end your discussion by asking "if something like this did happen again, what do you think would be an appropriate way for us to deal with it?" (In other words, talk about logical consequences before there is a "next time," but hopefully there won't be a need.)

How did he act out? There's acting out and acting out - did he speak out of turn, throw a temper tantrum or throw a smaller child out the window? For a bright and sociable Kindergartner to be sent to the Principal's office - that usually in and of itself is a punishment - bright children feel how demeaning that is to be banished from the community of their classroom and sent off to the Principal.
I don't know how his teacher handled it otherwise but as a teacher, I'd want to understand how it happened and what happened that caused a child to act out - unless a child has the habit of acting out, it's usually fairly easy to find what triggered the behavior and then to counsel the child to be aware of it and to be on the lookout for it so it doesn't happen the next time.
As a teacher, I'd ask parents to reinforce that kind of approach to the incident. No child is too young to be encouraged to be reflective about their behavior.