(2) The teacher and he informed two students for two minutes (1+2=3 people know).

(3) The teacher and three students inform four people, which takes three minutes (1+2+4=7 people know).

(4) The teacher and three students inform four people, which takes three minutes (1+2+4=7 people know).

(5) The teacher and seven students inform eight people, which takes four minutes (1+2+4+8= 15 people know).

(6) Teachers and 15 students inform 16 people, which takes 5 minutes (1+2+4+8+16 = 31people know).

(7) Teachers and 3 1 students inform 32 people, which takes 6 minutes (1+2+4+8+ 16+32=63 people know).

If 6 minutes, at least 1 people didn't notice,

Inform everyone for at least 7 minutes.