It will be interesting to see which side of the ledger this one comes down on.
There is the First Amendment and contract law to consider as well.
The definition of sexual harrassment includes behavior that is unwelcome.
The teacher blew the whistle on him, so I'm guessing there's more to it than just an essay. Something about it scared her and if his prior essays were sexually oriented it wasn't just that.
Did the essay identify the teacher or even A teacher? If so it might be harrassment. If not it might be argued a figment of his imagination. This was a class assignment. Did it meet the requirements? Lots of literature is sexually or attractiveness themed.
That said I had a lot of trouble focusing in HS when a certain teacher used to sit ON her desk in such a way that she would flash her panties to male students.
I guess it's an ill wind that blows no good.