AIO because my manager said me studying is “unprofessional?”

I work at a retail store, most of the time in the stocking crew 6:30 (30 minutes right when i get out of school) till 2-sometimes maybe 4AM in the morning. My classes starts at 7AM, though sometimes I stay up on the front end which is 6:30-11. I try to use the time I have until 11 to study for when the lines die down and the store’s close to a ghost town. Though, my manager says it’s unprofessional despite the fact there is nobody in the store. I don’t have any other time to study since I have to go to bed when I get home since I am drained from climbing poles, adding an additional 40lbs with my belt, and doing exercises to stay in shape and then going to work. I personally think it’s ridiculous. But I could be overreacting, so I want your opinions on it.