Shows how much I know...I never really thought about the fact that prefects and head boy and all that weren't simply invented by JKR for the books. I had never heard either of those terms until I read these books.
One thing to clear up, Lupin, James, and Sirius are all in Gryffindor. WEll about James a Head boy.. Have no idea about it. And in America, i never heard of thing such as Head boys/Girls or prefect.. Both public and private school. Well maybe in other states. But i'm positive there's such things in Ga. Probably you don't have to be prefect to be head boy/girl and Lupin might be Head boy too but wasn't mentioned.
I think like Harry is becoming a different person now, he is acting really different than in the previous books, James was changing also. He was of course always someone almost all the other students had respect for, and maybe after he'd matured a bit, he became more rule-guided? That could explain his position as Head boy. Though I thought I'd read somewhere that there are 6th year prefects also. So maybe he became a prefect in his 6th year, and Head boy after that. Wasn't Percy also a prefect in his 6th year?
It's self-explained since all 3 of them are so close together...Name somebody who are close together but in different house then i might be wrong about this.. . . And i remember somebody asked JK before on one of her interview or on internet when she was once on before long time ago.. And James can't be in Slytherin nor is Sirius and Lupin b/c they all hated Slytherin...That's why james picked on Snape is one of the reason.. James hates Dark Art and same goes for Sirius.
i think it works both ways. I mean, when Ron became prefect; Fred and George we're like "looks like we might have another head boy in the family"... nothing's definite
James was Gryffindor Chaser, pretty sure of that... even McGonnagal made comments in HP:PS