While few things scream “PROLE!” louder than not having a college degree by the age of 25, lack of a graduate degree doesn’t signal that much.
A large percentage of school teachers have graduate degrees, because such a large number of public school systems reward a bogus Master’s degree in Education with a salary increase or even require it to stay employed. And remember that school teachers are only a middle class profession, and closer to the prole side of middle class than the upper-middle-class side of middle class. Also, a lot of social workers have master’s degrees, and they fall near the same place on the class spectrum as school teachers.
Most people from the top-out-of-sight class have nothing more than a bachelor’s degree.
A graduate degree from a prestigious program or from any medical school is a marker of the upper-middle class more than the upper class, although it could indicate that the degree holder is a member of the working upper class. Or a graduate degree holder could be any upper-middle-class to top-out-of-sight person who enjoys school. Getting a degree just because you enjoy school and not because you expect it to help your career is an indicator that you’re higher than mere middle class and is also a marker of SWPLness.