Finally I don't have to dance around Haruyo.
There were a few hints this was a combined school — the most obvious being girls calling Momo "senpai" in chapter 4. Momo is a 1st year, so only middle schoolers would be saying that. Momo and Kuwa made indirect references to starting here in middle school too. And accessories like ribbons are commonly used to differentiate groups of students. We knew back in chapter 2 Haruyo was in a class with checkered ribbons. At the end of chapter 3 it's Kasumi's ribbon Haruyo focuses on, not the brooch. If you think about it, it's too coincidental for Haruyo to accidentally find Kasumi right after telling her to visit a specific place on a specific day. She walked by knowing it was her.
All that's to say you could have guessed this, but I didn't. These clues are tucked behind moments of high emotion or the same biased perspective Kasumi had of Haruyo. So when the reveal came I remember saying, "Oh wow," out loud.
There's a much longer conversation I need to have in the chapter 6 credits about register, the level of formality in speech. But Haruyo's language in the letters changes after finding out the truth. From flowery, to respectful with less elaborate vocab. She speaks that way too, treating Kasumi like an upperclassman. And Kasumi does the same, talking down to Haruyo once realizing she's younger. That's part of the shift in Kasumi's demeanor, from bitter wariness to comforting, though I think she's half blinded by her crush.
Mind you there's only a year difference between Kasumi and Haruyo; Japanese school systems are a bit different with 3 years of middle and 3 of high school. So it's effectively a sophomore x freshman. But it's more about the principle. The younger onee-sama putting on airs, the older little sister worn down by experience. (And that height gap.) That's what this manga is about: contrasts, anxiously guarded secrets, fantasies clashing with cold reality, and sometimes cold fears being blown away by warm truths.
In a sense, everything so far has been prologue. Building up the fantasy of "Onee-sama" to bring it crashing down last chapter and dismantled here into something awkwardly real. And while that might sound like a waste, trust me until you see what's in store.
CORRECTION: So uh it's come to my attention Kuwa wants to be in the FBI. See, in Japanese it's written vertically, and being an idiot I mistook the I for a dash and went hunting across Japanese internet to find what "FB-" stood for. Closest I could approximate was Family Business. Thankfully it's not incredibly major and I've updated it, but if there's any confusion that's why.