Option 1: 60% discount for students and full vote for teachers.

The cost is 7a+0.6×6a=7a+3.6a= 10.6a yuan.

Option 2: 29 people buy tickets.

Among them, 15 people enjoy a 30% discount.

Cost (20-15) a+15a× 0.7 = 5a+10.5a =15.5a.

The third scheme

It costs (7+6)a×0.8= 10.4 yuan.

Obviously, the third option is the best and the cheapest.