I guess it was the same in Poland and in America. It was never formally announced. Just sort of unwritten cultural norm.

Never heard of that in high school but my university's student handbook explicitly stated that if the professor did not show up within ten minutes of the scheduled start time, the class was officially cancelled for that day. I only remember that happening once, maybe twice, during my academic career. A few times they cancelled a class ahead of time but no-shows were extremely rare.