The problem is that it's obviously wrong with a simple experiment - just look at a few really old windows. I live about 1/4 mile from a church that was built 1300 years ago (https://en.wikipedia.org/wiki/St_Peter%27s_Church,_Monkwearm...). There's not much of the original glass left but where there is the thicker part is occasionally at the top of the pane because that's how it was originally mounted.

Teachers (and everyone else) will be wrong about things. That's OK. The problems happen when people don't bother to do any simple checks themselves, and just accept things as fact without doing any critical thinking or experimentation of their own.