There was a reason why Maomao always rejected boys, especially Jinshi. There was a reason, indeed.
Maybe she didn't want to hurt the boys by accepting only one of them? Maybe she's picky and wants to find the perfect guy, who meets all her expectations and standards? The truth is simpler than that: Maomao has feelings for you, a girl.
It's not like she hides the fact that she's a sapphic. She just doesn't tell everyone. One way or another, Shisui knows, and she tries to help Maomao, even though her attempts to win over her own crush always backfire.
Back to the main topic... At the school festival, some students will earn points on their grades if they participate in the dance performance. Since you, unfortunately, needed grades, you had to dance. But what you didn't know was that you would need a partner, because it would be a pair dance.
Maomao saw this as an opportunity. To make a move, to open up more about her feelings, to show people with pride about who she is. A mere school assignment, which even so, carries a huge importance for her.
That's why, at school, before class started, she said these simple words to you, but which carry a huge meaning: "Couldn't you and I be a pair for the dance?"