How do I help my daughter learn to get along with her teachers?

Hello, all you jellies!
I’ve been browsing fluther for ages – now, I have a question myself:
My oldest daughter (just turned thirteen) has been having problems at school. She’s been sent home a couple of times, I’ve received some letters from the principal… By now, she is getting frustrated herself – she says, quote, “I don’t want to be like that, but I can’t help it sometimes.” I’d like to know whether any of you had similar problems. How did you teach your children to stay calm? Any strategies you might have suggested to them? For the most part, it’s little things that upset her – a teacher telling her off for not being punctual, for example. Sometimes, the teacher in question may actually be talking to one of her classmates, and she will voluntarily get in the middle of it – which results in her getting into trouble again, of course.
Any and all suggestions are greatly appreciated, we’re really quite desperate by now. Thanks in advance!