Harumi is very knowledgeable about Aihara Academy even though she's only been attending since high school while most of the other students have been attending since elementary. It's later revealed that Harumi's older sister Mitsuko was the former student council president so Harumi learned about the how the academy operates from her. On top of that, the reason why Harumi didn't start attending Aihara Academy until high school is because she is deathly afraid of Mitsuko and probably didn't want to transfer until said sister graduated.
One popular theory posits on whether or not Mei was most likely molested when she was younger given her mannerisms and predatory ways towards Yuzu from the beginning. This is especially made worse when in one instance of trying to offer sex towards Yuzu after she learned that her father wouldn't return to the academy, she claims that she would be gentle as that was Yuzu's "first time."
Given how it's later revealed that Mei is mimicking the behavior of others, this further reinforces this theory. The "I'll be gentle" comment was likely said to her or something she witnessed before.
Well you need to imitate what others do when you are learning new things all the time and virgins (espacially teens) are often informed about sex through various descriptions and depictions of people doing it before they are doing that on their own. Mei could just simply learned how to have sex by watching sex scenes or other depictions of sex in media such as in movies (espacially the ones targeting towards youths) or by extreme cases through pornography which is quiet common among teens depending on area or country and it is very easy to find and look for whatever it is illegal or not. And also those two phrases are very cliche that could be heard everywhere in media that could be used by anyone no matter if they had sex or not.
There are some countries and places of the world where there's high number of teenage rapists and predators targeting at anyone from any age. These teens often have very poor social skills or suffers from mental illness but many of them became this way by the same way that most youths becomes bullies and school shooters, through neglect.
