...it really does raise the question of who outside knows what about the polycule.
I'd assume that the rest of class 1-4 knows at least that Rentarou is dating five of their classmates; it's less certain whether they know he's also dating one of their teachers, let alone the other girls. And probably other classes shared by multiple girlfriends have noticed that they're suddenly more chummy, as seen here — the classes with Ahko and Suu, with Iku and Yamame, with Rin and Uto, and with Kurumi and Momiji. And certainly outsiders can readily find out a fair bit about the Family, as the school newspaper reporter demonstrated (and as will be shown with some later girlfriends). They're relatively public — as was already mentioned, they take part in various events and competitions openly, and at any rate aren't exactly hiding. That said, the situation has also been misunderstood before — like when Uncle Hiro didn't realize Rentarou was already dating the girls he was suddenly surrounded by, and there are some future examples as well.
The situation is no doubt complicated by various harem members' fame within the school. We've seen other students talking about Hakari, Nano, Mimimi, Meme, and Momiji so far, and obviously lots of people know the faculty members.
(Then again, we don't even have a full picture of which of the girls' families have what information about the situation. Obviously the Hanazonos, the Iins, and the Yakuzens all know exactly what's going on, but the list of which girlfriends' families even know they're dating Rentarou isn't clear. Hmm. Something to track on my next reread.)