Is it illegal for headteachers in the uk to look at students FB acounts?

[Amateurfaucet965]

Not Illegal.
He didn't hack into them.
The pictures were probably copied from looking at an acount whose privacy settings were not set to private - while yours are another students might not have been, or he might be friends with another student allowing him to see photos.

Rule of thumbs for online and social networking:
If you don't want me to know, then don't put it online
Nothing is ever deleted online, even if FB deleted it, people copy information for their own use
Once posted, you can't control the information or photos.

If you are worried about doing something wrong, then don'[t do anything wrong in future