I definitely agree, as it would be fun to talk to other players in a party chat while you are in class, versus just constantly messaging them.
Adding on to this, I think it's mainly because it's recommended you leave all channels in a class in order to focus on the notes and not lose track of what the class is about, otherwise you won't do well on the assignment. That's probably why players are not allowed to message in global while they are in class, as it will distract them from the class