Egads - found the time to talk with the teacher, and with my daughter, too - turns out the situation is a lot less worrying (good, that), but also a good deal more complicated (ehh..).
Basically, the heterosexism bit still stands. The teacher was trying to warn my daughter against stupidly getting herself into trouble, which is, I suppose, a good intention. But I'm still uneasy as I really really don't like the implications of a) that my daughter shouldn't provoke the frilly brigade by being herself, and b) that can't we all just get along, but queerish people please give way and be the smarter types. It's all very practical and that, I can see the point, but I'm afraid I'm getting more and more irritated at being accommodating to other people's prejudices.
It's really a double bind: if you accommodate, you're erasing yourself, and if you don't, you're provoking trouble and are labeled with a pejorative label of their choice. Bugger that.