Is it legal/ethical to include photos of my students in my classroom online (as in my online portfolio)?
In the US, it would violate FERPA to identify students as having been in your class. I assume other countries have similar laws and I think most would agree it is a violation of their privacy (i.e. unethical) to do so without permission.
Talk to legal counsel at your university and do everything under their guidance, which would probably involve a signed release by each student.