No bookbag because the bookbag is on you. No holstered on you because then you're carrying concealed and need a permit to do that. You can keep the gun ready to go but just put it in a compartment. Don't crotch rockets have compartments?

While you're in a car if the holster snaps (read here, SNAPS) around the gun and prevents it from being used until you unsnap the holster and remove the gun from it, then you can put the gun in that and put it under your seat or something. OR in a car you can stick the gun in your center console or glovebox, or keep it in a box, or a zippered bag that it can't be simply fired from within. Important part is that the container renders it unusable until removed from it, and the container closes somehow.