That is a tough question and one I have avoided asking about really – if I want to use pics of students at school I try to use ones that do not show the complete face, but are side on, looking down or just partials. I certainly do not attach names to the pics.
Would this be the same as posting pics of complete strangers that we take photographs of that we use on our own sites elsewhere such as flickr and instagram that are taken & used without permission? Or is it more than that? Ethics of online images ….