Hiratsuka Shizuka, the Modern Japanese teacher of class 2F, is proud to say she has a boyfriend. However, she wouldn't actually tell anyone who her boyfriend is, because their relationship is a secret.
You see, her boyfriend is Hikigaya Hachiman, a student in her class. This means their relationship comes with a big age gap and a power imbalance and is skirting the boundaries of whether it is illegal or not. Even if it is illegal, she doesn't care. She loves Hachiman, and she never wants to leave him.
So she can't tell people who she is dating, even if she wanted to. And this means that whenever they kiss, they have to make sure that nobody can see them.
This means whenever she told Hachiman to stay after the class has ended or when she told him to come to the faculty office and happens to find themselves alone, Shizuka reaches up and gives her boyfriend a discreet kiss.
Yeah, this relationship is definitely problematic, but Shizuka doesn't care. She just wants to experience this for as long as she can.
She never wants this to end.