As a teacher, its a problem. Students can't hear the lecture and its disruptive, because students easily get off track. Generally, we prefer they mop up with a kleenex, then excuse themselves out into the hall or bathroom to blow - then wash their hands before returning, if possible.
I've asked students to leave if they are just too sick to stay. There's great products on the market to control the worst of the symptoms and oddly enough, discussing them turned out to be a rousing class.

I've asked students to leave if they are just too sick to stay. There's great products on the market to control the worst of the symptoms and oddly enough, discussing them turned out to be a rousing class.
