As a grad student, I had a professor mentor talk about what he called "classroom erotica," which was the unspoken sexiness of smart people learnin' stuff together or from each other. He was, however, talking about legal adults (college students and professors), and still telling us why it would be unethical to act, at least while said student was in our class (power dynamics).

Now that I teach high school, this whole idea makes me a little sick to think about. Does it still apply? Do some people feel this? Maybe. Most people can recognize attractive qualities in their students, and many of us may play the "if I was a student in this school" game in our heads about all sorts of things. But here's the thing: you are an adult and your student is a child that you are supposed to protect. Even if you feel it, or if a student feels it, you need to shut that down immediately and be the responsible one. Otherwise, you cannot be a teacher. You have too much power over these students, even if they are not in your class, to ever have any kind of fair two-way relationship. It is always a power dynamic when they are a student and you are a teacher, and it is always you with the power. Your image of a passive teacher/aggressive student simply cannot exist because the teacher has the power in the first place. "Letting it happen" WOULD be either aggression OR a sexual assault committed by the student, depending how it went down.

You're having a hard time with this because you already believe your friend from school is a good person. You're having a tough time accepting this new information that maybe he isn't, so you're trying to rationalize for him. It's good that you want to still see him as human/your friend, but you need to hold him accountable and realize what he did. He preyed on a vulnerable child. The fact that you show so much sympathy for the teacher and seem to show pretty much none for his victim worries me. You're imagining his victim as a sexual aggressor, even though you know he sent the inappropriate messages. Even if she initiated it, again, he is the adult who needs to see that maybe she needs to see a counselor, not that he should play along. You need to see your students (all of them) as worthy of protection from the adults who have been entrusted with their care. If you can't see that, you might consider another career.