Answer: the first minute notice 1 classmate;
You can notify up to 3 students in the second minute;
Up to 7 students can be notified in the third minute;
The fourth minute can inform 15 students at most;
At most 3 1 student can be notified in the fifth minute;
3 1>20.
So it takes at least 5 minutes to notify each player.
Comments: The key to solve this problem is to use the number of students already notified plus the number of students to be notified by the teacher in the next step.