Today was a bookBook Day, a day that students got to wear something that relates to a character ofrom their favourite book.

I expected people who isit to only be people in kindergearten or primary school who were wearing something, but actually people who is in high schoolers were wearing somethings too.

People = plural = are, not is

People in high school --> you can just say "high schoolers" (also kindergarteners and primary schoolers)

Fun fact - kindergarten means children garden in German (kinder = children, garten = garden). I always think of this when I see the word kindergarten.

My oOne of my friends was also wearing all black clothes all over her body, and I asked her which book did she come up with thatget the idea for those clothes from, but I didn't really understand it her answer.

All black clothes/clothing = all of her clothes were black

"All over her body" sounds kind of awkward, I wouldn't use this phrasing.

I wasn't sure what you were referring to with "it" here, so I tried to make it clearer.

Any way, I am going to get up earlier than usual to get more time to study because I didn't enough time to study mathematics.

It would sound more natural if you specified when you are referring too in this sentence --> ie. I am going to get up earlier than usual tomorrow...