If you are on the NTR route you have currently lost this ability. Future NTR updates may solve this.
Main route: If you haven't done it in a while, you go to school, go to the men's changing room. You will see 3 non interactable shota like art pieces. Next to them on the right is another door that leads to the bathrooms. If you do in there and lick on a stall you play a stupid minigame to basically jizz in a jar and make money.
Thk