OOP: prefects & head boy

[doseylel]

No: HPFGUIDX 61424

I'm not sure I understand the process of selecting Head Boy and 
Girl.  I was under the impression that head boy and girl were chosen 
from the prefects.  We already knew that James was Head Boy, but in 
OOP we learned that Lupin was made prefect in 5th year.  Of course, 
if they were in different houses, this wouldn't matter, but I've 
always pictured MWPP as being in the same house.  Am I way off base?