Never an excuse. When you're moving between classes, you're old enough to have lockers. My high school was huge and 4 minutes was plenty of time to go to your locker (even if it was all the way across the school) exchange books, and go to your next class. At most you should have to carry two classes worth of books.
I couldn't get from our gym to the second floor class rooms at the back of the school in the 3 mins we were allotted, let alone get to my locker on a different floor on the other side of the building. Our school was built long, not circular.
I certainly didn't have enough time to use the restrooms during period changes AND go to my locker to get my books and other supplies unique for that class AND get to the class in 3 minutes.
And heaven forbid if you don't have extreme OCD on keeping your locker organized and you are just fucked if you are a new student or your schedule changes and you have to relearn it.