The first minute: the teacher informed 1.

Minute 2: Teacher Notification 2, 1 Notification 3

Minute 3: Teacher's Notification 4, 1 Notification 5, 2 Notification 6, 3 Notification 7.

The fourth minute: the teacher informs 8, 1, 9, 2, 10, 3,1,4, 12, 5, 13, 6,/kloc-.

And so on: the number of people who can be notified per minute is 2 (n- 1).

The number of people who got the general notice was 2 n- 1.

Make 2 n- 1 ≥ 28

The solution is n ≥ 5

In other words, all notifications can be completed within 5 minutes.