jellybelly, the intentions of the teacher are neither here nor there. If I posted an embarrassing picture of you to the local paper because I attached the wrong file (was supposed to be "Badgers seen in supermarket car park!" but was actually "jelly belly tries on unflattering swim suits") then no matter how many times I apologised it wouldn't make it less embarrassing. maybe you wouldn't care how many people see you in unflattering swim suits, but if you care, you care, and it doesn't matter how or why they have suddenly been shown to your neighbours, your workmates, that guy you fancy, your children's teachers, etc.
Something I often think about in arguments on here about facebook and photos and privacy is this: the accessibility of the image or the information doesn't make it fair game. We all know this with letters and printed photos. If I was at my friend's house and she had some opened letters in envelopes on the side, it would be really wrong of me to get them out and read them - but I physically could. If I was at my friend's house and she left the room for a few minutes, leaving a photo album out on the coffee table, it would probably be fine to flick through - but really wrong to copy or take the photos to give to people who wouldn't have been invited to her house (or in other words, leaving a photo album out is a tacit invitation to friends and family who are naturally present in your house to have a look, but you wouldn't take it any further).
the fact that it is physically possible for us to violate people's privacy in all sorts of ways, in daily life, while interacting at normal levels of trust and good manners, is really not usually an issue, because most people just don't do it.
Yet, this bizarre argument is always being applied to facebook: it was possible, so it was fair game.
Now the teacher would have been right to keep pointing out what is possible. But I think (s)he was really, really wrong to perform this violation. yes, because it hurt the girl. But also, because it is a really bad argument, an argument in favour of bad manners and cruelty: if it is possible to hurt someone, it is ok to hurt someone.
Society simply can't operate like this. And, on the whole, it doesn't. Most of us go through life automatically taking great pains not to hurt each other, through conventions we have internalised. Some of these conventions relate to privacy.
teenagers aren't quite there yet. Some are lovely, most are trying, but en masse they haven't quite got there with the level of self control that is required to be a decent adult in society who generally do not cause offence or hurt to anyone. This teacher has really messed up an important part of his/her job in socialising teachers: learning that just because you can hurt someone, doesn't mean you should (even if it is funny) is one of the most important things teenagers should learn.