Daughter will wear a coat most cold days, but no one uses their lockers. Hers is upstairs in a corner and when it is time to catch the bus she doesn't always have time to run up to get it then down to the band room to grab her instrument and then out to the bus on time. So I think she usually keeps her coat in her girls' locker room locker.
They do say it is "safe" to keep things in your locker because they have hallway cameras if anything was ever stolen. But I think the majority of kids carry their full backpack to each class. One thing that is different is most textbooks are optional now, so they aren't carrying around a ton of books. Texts are either online or teachers don't use them. She decided to get the algebra textbook and the science textbook, but she keeps them at home.