That's what the coaching function is for. You just need to become their coach. I'd show you a guide but I'm currently using a tablet so I can't, but since it's one of the main features on Khan Acad...

1. Közösség
2. Classroom Tips and Tricks
3. Mark questions or sections for students

I agree with Adam. However he did not cover the "Plus, please do not put topics I have already Mastered in Mastery Challenges." part so I will. If you, say, master long multiplication in a week and...

1. Közösség
2. Feature Requests and General Site Feedback
3. Mastery Challenges/Practice tasks