I can't change my class

[DaCheese]

I'm trying to change my class so I can follow the guys in Raisins, But I can't change class, I have also finished the game

[Macandchess]

you need to change into the different hero costumes.

[Invader Zim]

Select Powers, scroll to the Super attack of each Class you need to be and select for each follow. Costumes have absolutely nothing to do with it.

[Leodrawde]

At some point (during Conners/Cartman's riddles), Timmy gives you access to ALL classes.
You just need to change to one power from the class(es) required by each follower. As simple as that.