Yeah but completely ending the relationship like that is a little too drastic.
On the one hand, yeah, she's being a bit dramatic. But on the other hand, you can see how quick Bocchi is to give up if there is any chance of escape.
Plus her wording on the terms are a bit vague. To further explain, as far as I can tell, the cutting off of relations lasts until they graduate high school, and is in effect for as long as Bocchi isn't friends with the whole class. Meaning theoretically even if Bocchi completely gave up they could supposedly make up in high school.
Stipulating the entirety of the class was surely far too drastic, but that's also really one of the main ways to keep the manga going.