You can say, I thought you were a teacher.
This can either refer to the present or the past. The real question is, just what do you intend by this question? If you are referring to the present the possibilities are thus:
It may be that you are saying:
Oh, I THOUGHT you were a teacher. That is, I am right after all; you ARE a teacher.
Or it may be that you are saying:
Gee, I thought were a teacher but you evidently aren't.
Context is everything here.
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