They can be girls if they want, and I don't really care their reasons. But if their reason is to be cute, the reality is really harsh. If they don't take care of themself (except the lucky ones), every month there will be a period, when everything will be bad, like very bad. There will be pimples in some part of their body (for me, when I was teenage it was the forehead, back, and (after I had those) beetwen my breasts. There will be 3-8 day of suffer, when they should frequently go to hte batheroom for change. It will ache under their stomach (if they are lucky, it will be short, and weak ache, but if don't....), it will be feel bad to do sport, but have to, because they are in school, and have PE. (And the period will be better, if you do sport frequently) And these are the superficial things.
Change gender if they want, be changed if they like it, I don't care. But they should fully understand the downside to be a girl. And I don't think they will apreciate it, because this show them, that they can give birth.
I think everyone will kill him together at their first period. Or beg him to reverse this. If they are lucky, it won't hurt much, or long. Just a day or like.
I wouldn't like to be nurse in that school.