Yep! Ms. Shiimada, (I think that was her name), is transgender, and dating Kei’s aunt, who is the school nurse.
She more or less just gave Yuuki the green light to dress in a girl’s uniform, so he can “be himself.”
@Draken
Well, that may be part of it, but I don’t think she knows about his presentation choices.
She knows Kei had a job, and was comfortable in it, but I don’t think she met him before.
It's worth pointing out that peer pressure is a real thing while at school. It's ok to be yourself as long as you're not very different from everyone else. In many ways Yuki would be better off trying to fit in until he graduates.