I also have to say, when teachers do that, and say, well evolution is all just a theory, it's not proven. It's just a theory.
It really undermines the scientific method. As others have said, gravity and plate tectonics are theories.
I don't think its right for an authority figure or instructor to act like that. There are people who study the theory of plate tectonics, understand it, but don't understand that it's a theory.
I think it's better for teachers to say nothing than to obfuscate technical terminology.