Only way I found to get xp from combat is to fight the guards on the way to the captains office (So stock up on apples and stuff so you can farm those halls) and fighting the guard in front of the elevator which can only be done once a day. Otherwise doing quest but there are only so many of those. I honestly think it's a bug and all the exp is going to the inmate/guards after the fight because they jump from level 0-4 even if you beat them.