Aussi têtu qu'une mule

L'un de mes étudiants est aussi têtu qu'une mule. // One of my students is as stubborn as a mule.

I think the cat has a pretty good idea as to which one of the students his colleague is talking about.

When we were still at the uni all these years ago, one of the students was amusing himself by writing the central statement of Fermat's Last Theorem on the clean blackboard sometime before the math classes started. I think our professor was under impression that we had a budding "fermatist" in our midst, which we didn't of course.

The Last Theorem has been proven since.