In a school, the ratio of boys is to girls is 9:4. If there are presently 720 girls students, how many more girls would it take to reduce the ratio of boys to girls to 2:1?
Given that the ratio of boys to girls is 9:4, there are 9+4=<<9+4=13>>13 parts in the ratio.
The number of boys in the school is currently 9/13*720 = <<9/13*720=495>>495
The number of girls would need to reduce the ratio to 2:1 is 2/1*495 = <<2/1*495=990>>990
There would need to be an additional 990-720 = <<990-720=270>>270 girls. Answer: \boxed{270}.