What is an appropriate punishment for a 5 year old who gets in trouble in school?

My 5 year old son has had a little bit of trouble in Kindergarten. I really don't understand why, because he loved Pre-K. Anyhow, today he came home with a note that said he dumped his drink over his clmates head. I have been trying really hard to get him to follow the rules at school. I want to punish him for this, but I am not sure what an appropriate punishment is, or how long it should last. Right now he is in his room thinking about what he did (he ended up falling asleep). Any suggestions on what an appropriate punishment is & how long I should punish him for would be helpful. He is my first and only child, so I am new to punishing for school issues. He will have time out at lunch tomorrow instead of recess, but I don't think this will solve the issue. Help!!