The second minute: the teacher and classmates inform together, so that the notified person can continue to inform the unknown person. This time, two people were informed that three students knew that two people were informed.
The third minute: by analogy, four people were notified this time, and seven students knew that four people were notified.
Fourth minute: 8 people were notified this time. * *15 students know that 8 people have notified.
Fifth minute: 16 people were notified this time, ***3 1 classmates know 16 people were notified.
Sixth minute: 24 people were notified this time, and ***45 students knew that 24 people had notified.
The number of people known for the second time is equal to the number of people known for the last time.
Plus the teacher, if n people know at first, 2n people know in the first minute, 2n people know in the second minute and 3n people know in the third minute. ...
Only the teacher knew at first, so n = 1 because 2 5.
So it only takes six minutes.