Very well done!
I enjoyed reading this story very much.
It was not like reading a text - but in my mind as watching a movie.

And to top it:
Only after reading I saw the pictures that inspired this - and the one with her inside the locker was just as it looked like in my mind reading the text.

The plot develops well thought out. Only when the uncle mentions his nephew having been locked into a locker you first could get it would end up for her in a locker - as it is a lesson on her as a revenge.

The point reading it is that it's from the view of the uncle. You only know what his nephew told him. You don't know if this is the truth. Or not. Is he right his teacher is flirting with students? Is she right telling she won't do it?
And it's hard to judge it. On the one hand she is making sure not to show her body - on the other hand considering what she's wearing under the top clothes...

An open end is always what makes a short story. We don't know who will find her.
Let's hope for her it won't be the students but someone else before - since we don't know whether she earned this evil revenge or not.