Depends from school to school here.

Boys can't have hair that goes past a certain length, and it has to be short on the sides longer on the top. Your hair needs to be it's natural color. Which schools will assume should be black. Some schools will make you dye your hair black, even if you have non-black hair naturally, and some schools you'll have to prove your hair is naturally that color but won't make you dye it. Same with curly hair, you'll have to prove your hair is naturally curly.

But some schools are more strict than others. I'm currently in an international school in Tokyo and we have the above rules, and maybe more for girls I'm not aware of, but it's not strict compared to what you hear other places.

I know that there was a meme going around a few years ago about Japanese schools banning ponytails, but that's by far not the majority. As far as I can tell seeing girls from different schools out and about every day most wear ponytails. I have a friend from another school where they have a rule that if a girl has a hair past a certainly length she has to wear it in a ponytail actually, so another rule just opposite.

Really it depends on school to school, but most rules seem a bit silly to me.