I think it's a couple of things. The most selfish is that if they're not actively profiting off you, they feel resentment just for you being there. Outside of that, it's taking time away from other students you could be teaching (although in reality having more advanced students should only make teaching easier, but I guess that depends on teaching style), and depending on how crowded the class is, they may be running out of actual room. I've seen classes with 30+ people in there, and each person takes away space and makes the room that much hotter.You hear stories like this Way too often. I don't get it at all. It is not like any school is running out of physical room and a warm body is usually a good presence every way imaginable. It takes a really big pair to try and pull crap like that.