You should just be lucky that Hei isn't going to lust after "jail bait" because, technically, Hei is nineteen and while she doesn't care about have student-teacher relationships, she isn't going to go after kids.
Plus, I'm pretty sure Hei has a crush on Cynthia. ;) And that's weird, considering that Bai and Cynthia have a sister-mentor(ish) relationship and Hui just admires Cynthia's intelligence.