Yaoi oneshot, may have been part of a collection.

Begins with a student seeing his teacher coming out of a bar or hotel, and the teacher is dressed provocatively and is a slutty uke?

Various other details are fuzzy, the teacher may have had longish dark hair and likes to pick up men. The student may have tried to blackmail him. But they do have sex. Because they always do.

Okay, I'm guaranteeing that this isn't what you're looking for, but when I saw the description, I was reminded of this oneshot. It's kinda sorta similar to the one you described, but not really. You may be interested in checking it out anyway. Hot Closet.

Mind eraser, no chaser

Wow. Faints

Bump? O_O
Anyone?

A bump.