Well, as we disagree on the amount of students the school has, this is kind of a hard discussion ! I believe that there are more or less 300 students. 24 in 300 would be quite enough I think (is there a place where one can find the police - people ratio in counties?), 76 would be too much. That's 1 in 4!
Sure, for most years the HB/HG will be ex-prefects, for the reasons you gave, being exceptional students and stuff, but if there is a student who has suddenly changed his/her ways, I don't see why the Headmaster couldn't choose that student!
Oh, by the way, how do you get to 72 prefects?! The 24 I can calculate, but the other one not! Thanks :Þ