Why is school so exhausting?

I dont understand why it is so exhausting. Nothings happening, im not being picked on anymore, I am happy with most of my teachers.

I come out of each lesson exhausted and wanting to go home, I want to interact with my friends but its really hard to do when im just too tired to do it. Im tired of putting up a front and pretending everythings okay, but I dont want to be vulnerable because what if this is just something which is just happening at the moment and it goes away. I dont want to open up about something just for it to go away within a week and cause unnecessary worry.

I cant even take a day off because ive already had too many days off this year and I dont want to fall behind.

I cant even catch up on rest on the weekends because i want to get back hours to myself that I missed from being at school.