That's definitely true, at least for my School (Europe) when you turn 16 (or equivalent in school year) you can choose your own subjects to take rather than mandatory classes. So things like Physical education could be dropped, while Psychology could be picked up to replace it's slot. Denji is supposedly 17 by the way.

Although In Japan, from what I've read Physical education is Mandatory for all of school, though they can choose other subjects except for mandatory P.E. Would be completely in Denji's character to skip class, but He did say so himself he wanted an education but couldn't due to being forced to make money to pay off his dead father's debt to the Yakuza from a young age forcing him to work (I.e Chapter 1). Fair enough he gets plenty of exercise killing devils as CSM though, so perhaps that's fine in his eyes. Then again, P.E just might not exist in this world, at least the P.E of our real world anyway. Considering devil hunting classes exist and all.