Well to be honest you shouldn't be taking photos of random people :P.
But about the bird thing, he has no right to influence your opinion of what you're taking photos of unless there is a specific criteria. I know for a fact that my Photography teacher doesn't care about what we take photos of, more just the qualities we captured.
You should tell him that he isn't allowed to bring person opinion into marking or judging your work, and if he does, I'm pretty sure you can say something to the Principal and get him shaped into line.