This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Once a Teacher, Always a Teacher side quest.
Time frame: From Android Terror Arrives - Episode 8: Fleeting Normality to Episode 8: Kami's Successor
Reward: 121,766 EXP, Super Sacred Text of Martial Arts, Fancy Wristwatch, Card 042
Where to start: Kami's Lookout Area, as Gohan.
Go to Kami's Lookout. Piccolo is waiting for you there. Talk to him to start the mission.
The only objective here is to have a sparring with Piccolo. The opponent will either be at the same level as yours or slightly lower. He shouldn't cause you any trouble. After defeating Piccolo, talk to him to complete the mission.