Bugger me, Chapter 100 is right around the corner. Also, does Kei's getup meet school regulations? Given how strict Japanese Schools can be, I reckon Mitsuki's attire is pushing it. Even though she looks so boss.
I vaguely remember a manga from a while back where one of the characters wore the opposite gender’s school uniform by abusing the loophole that their school’s regulations only required the students to wear “the” school uniform, but didn’t specify if it had to be the boys’ or girls’ one.
A few years ago I read an article about some Japanese schools intentionally changing their regulations like that in order to become more accommodating for trans students. Some schools went even further by only requiring students’ school uniforms to consist of the approved items, so students were free to mix and match as they wanted. Apparently, the girls’ blouse and blazer with the boys’ pants became a popular combination for girls in winter, to the surprise of absolutely no-one.