Nagisa may be unused to drama of any kind. These are teenagers. Could have been going "??? DOES NOT COMPUTE, OUT OF CHEESE ERROR" at what Glasses was doing.
Similarly, Mashiro has never experienced anything like this: I don't think she's like thinking she needs to break up with Nagisa, she feels like the cocktail of jealousy and anger is kind of an awful feeling and wants to be away from the sources of these emotions right now. She needs time to herself to cool off and process what is even happening to her right now (whether she does so or not is another story).