Some of you seem confused by the transition between chapters? But I think it's crystal clear! Yes, the author skipped some parts because she didn't want to waste space in obvious and uninteresting stuff, but that isn't really a problem: the narrative is smooth.
Makino took the credit card and bought an engagement ring (for Mashiro) ==> Whatshisface thought his marriage with Makino was a go ==> He told their parents that Makino had agreed to marry him ==> Makino's mom went to the cram school to arrange her withdrawal ==> Makino's mom learned how a left hook to the jaw feels.
The not-shown parts are the boring bits between the different stages of the narrative development... like Whatshisface, his parents and Makino's parents discussing the marriage plans while Makino in the background jumps up and down and yells that it's all a misunderstanding but nobody pays attention to her. We don't need or care to read about that, we can guess how it went. Moving from the end of last chapter to the very important climactic developments in this chapter is the right narrative choice.
Speaking of important stuff, one thing that hasn't been shown yet but we definitely will get to see next chapter is the chat between Makino and Mashiro about the episode at the jeweller's! Mashiro will demand to know what was going on at the time... and after Makino explains I suspect Mashiro, faithful to her role as Makino's Jiminy Cricket, will tell her to return card, ring and everything to that guy. "You should buy me a ring with the money you earned yourself, Makino-chan!" Or something along the lines.