Is this the scene you were looking for? It's from a 70s film called "Intimate Games" which is a British softcore film. It's not quite what you're describing, because it's not a teacher and a student, but actually two students collaborating on a project for class. But it does feature a girl getting frustrated with a guy's inability to focus on what's she's saying and deciding that the best solution is to just get naked.
It's in this thread if it is the one you're looking for:
https://forum.oneclickchicks.com/show...4&postcount=17
johnycancer