with public punishments
The more discipline you apply publicly, the higher your discipline rating. Explore everywhere every time you can, it's not uncommon to find situations in the halls and gym entrance that can be used to increase discipline. Also watch out for situations that can increase your popularity at the cost of discipline. Sure, the increased popularity might seem nice, but not at the cost of discipline that sometimes accompanies it, such as walking around on the weekends and not telling the girls to keep the volume down on the music. Once your discipline gets high enough, you can go back to working on popularity, as you should already have enough discipline to unlock new rules, which in turn grants more discipline points.