"Sort of homeschooled," says Bella. "Why are you concerned with my boyfriend's education?"
"How does their being enrolled in a public high school help one know about them?"
"Minnie," says Bella, looking around and finding that there are no nearby witnesses, "are you by any chance a vengeful ghost who kills people who go stag to prom?"
Bella hunts down Sherlock, who is presumably still in the middle of dancing with Kayli. "Hey sweetie!" she says. "Can I cut in? I found Minnie and we should go say hi!"
"Certainly," he says, giving Kayli a final twirl and bowing over her hand as he steps back.
[She went through that wall,] Bella adds, and she describes what Minnie looks like, [presuming her appearance is stable, anyway. She talks like a person, up to the point where I got her name and asked her if she was by any chance a vengeful ghost - I don't know how reasonable she is, she laughed and backed away when I said that.]
[I see,] says Sherlock. [I wonder if I should hang around the afflicted sink. She can't exactly drown me.]
[Good idea. Here - you should be carrying squares anyway -] She unclips her chain and shakes four of the coins into her hand and presses them into his, then clips the chain back on. [I'll go around and see if she's in the room she appeared to go to, though I'm not betting on it.]
That wall leads to a science classroom.
The ghost is not there. Or at least is not visibly there. "Minnie?" Bella tries. No luck.
She heads back into prom, scanning the crowd for that face, that dress.
Okay - headcount. People were only admitted in pairs. Is there an even number of people in the room?
[Is the coast clear by you?] Bella asks Sherlock. She goes to check the other bathroom; if two people have gone to the bathroom at the same time that would also yield this mathematical result.