Whenever my DD7 is being especially touchy and sensitive with the little issues, I've found that it's a canary in the coalmine for some big issue (at least to her) that she's been reluctant to talk about, for one reason or another. The last time I watched her blow up all weekend, I finally coaxed her into a conversation, in which she revealed a boy had been harassing her constantly on the school bus. She hadn't bothered talking to us about it, because she thought we couldn't solve the problem. She went back to being a car rider (she only started riding the bus on her own request), problem solved.
If the "big issue" in this case is an ongoing pattern of problems with the teacher, threatening to punish him at home would have done more harm than good.
