Well, I've heard that JK reckons that there are around 1000 students at Hogwarts... I know that this doesnt' add up if you calc out on stuff like the boys in Harry's grade, etc, but if that's what she's said??? So with two from each house and yrs 5, 6, & 7, that's only 24 prefects for 1000 people?
Also, about beign prefect to be HB/HG... my logic would just be that Prefects are suppposed to be the most responsible, most likely to keep everyone else in line, etc from each grade, no? So, it would just make sense that HB/HG would be chosen from those???
The other idea I've brought up before it if new prefects are chosen each yr... ie,the ones from 5th yr are kept, plus 2 more from each house in 6th yr, 2 more in 7th yr? that woud only make 72 prefects for 1000 people... But the main reason I think that somehting like this will happen, is after JK made Draco (and other 'popular' students that we know already) prefects, I dont' think that she will leave Harry out...
(I think that my refusal to let go of this is kind of akin to my refusal to let go of the James didn't die at GH theory!!! I just cant' help it!!! )
Anyways, I've finished my rambling now! So feel free to continue!