This is a teacher in school during the school day.
Yup. So what? In private instruction? With nobody else around? Who cares that it is “in school during the school day”? Do a little Googling — happens all the time.
You really don’t understand how evil some people can be, do you? And how easy an adult male in a position of authority can take advantage of an adolescent?
Again, this teacher could be pure as the driven snow, but he MIGHT NOT BE. Given that the OP’s niece apparently has not raised these red flags before, why take the chance?
I can tell that you really don’t understand, so this will be my last response to you. To the OP: Please consider what I said in #24.