Hagrid's blunder or can any student by head boy/girl

[prongsnpadfoot]

No: HPFGUIDX 93914

Hi All,

While listening to PS again (uk version),just realised that Hagrid 
tells Harry that his mother and father where head boy/girl at 
Hogwarts in their time, sorry don't have the book to hand.

But in OOTP, we are told that Lupin, got the prefect's badge, not 
James or Sirius. 

This could explain why Harry though his father was prefect, and was 
relieved to find he wasn't.

So can any of the students be head boy/girl, or did Hagrid have his 
facts wrong?

Dan