How far down the rabbit hole are you willing to go before you decide, "No, that's 'secure' enough?" What if she had the passcode 1234? What if it was a really easy pattern?

If she made an actual effort to make sure students didn't get into her stuff, then she'd be absolved of all responsibility.

It was her personal device that no one should be accessing. That's like blaming the teacher for keeping her unlocked laptop in her car and not rolling up the window fully. Yes it's stupid, but only stupid because of the potential loss of property.

This analogy doesn't work because the teacher has no responsibility for the well-being of random thieves off the street. She does have a responsibility to the students in her classroom. She wasn't merely reckless or naive. She didn't live up to what was required of her. If she had prescription medicine in her purse and a student stole it, she would bear some responsibility for anything that might happen.

It's not criminal for her to possess the photos and it shouldn't even be mentioned that she didn't lock a personal device.

No one said it was criminal. At worst, it was a minor violation of school policy. I think she should have been given a verbal reprimand and told to either use a password or lock her phone up, but she still brought unsecured pornography of herself into school.

It doesn't matter if it's a good idea to lock it, she shouldn't be required to encrypt her personal belongings because some lawbreaking individual might find things they "shouldn't".

A number of minors never would have seen her nudes if she hadn't intentionally brought them on to school grounds. If she brought 10x15 printouts of her vagina in a briefcase, no one would bat an eye if she was fired after a student found them, regardless of the student's intent.