Can't figure out this story just from glancing at the images. First she has no belt, then she has one, then SHE has the key, then it's off, then it's on... And then there's this second girl all of a sudden. And where did the guy go?
I Google Translated some bits from chapter 4 and beyond and it's some kind of "teacher rewarding her student for getting his grades up" thing where she buys herself a chastity belt because his grades have fallen but she no longer trusts herself to hold to back from having sex with him. It's on then off then on because she orders, receives, and tries it on alone, gives in to the temptation to pleasure herself one more time before committing to it, then springs it on him the next day and tells him they're going to follow the advice she got from a chastity blog and seal the keys in an envelope that they both sign. (Even as she admits to herself that she's used to masturbating once or twice a day.)
The pages with the second girl are a repeated "end of chapter" page with text that's breaking the fourth wall to speak to the reader.