"I didn't do anything." It should be a lot easier to lie to a mindreader. Of course, lying to Professor X should feel like lying to your oldest favorite aunt who is on her deathbed and can go at any minute... no, not really, but it should feel pretty bad.

Kate is pretty terrible at outright lying anyway. She does a lot better with simple omission of facts, but regular lying shows right on her face. Those red cheeks aren't just there for decoration.