At nearly every
American University/
College, there is the
unofficial notion that an
instructor can be exactly 10 minutes late before class is cancelled. This is nothing more than
urban legend at most institutions, but if all of the students in a class decide together to leave after 10 minutes, you can be sure that the class will be
cancelled.
In all actuality, it is a toss-up as to whether or not a student can be
benefited by executing the 10 minute rule. If a class is
officially cancelled, that time must be made out at some other point during the
semester. Therefore, if the prof shows up after 11 minutes and the class has to be cancelled due to the walkout, those students may get stuck coming in an extra day or spending some additional time in subsequent
lectures. If the prof never shows up, of course you want to get out of there asap so that you're not stuck wasting that class period plus the time you'll have to spend making up the cancelled class.