In Harry Potter and the Half-Blood Prince, we learn that Ron and Hermione continue their prefect duties as sixth-year students. We know that they were awarded this position in Book 5.
My questions are: Are there 4 prefects at anytime? Two as 5th years, and two as 6th years?; I know that there is a Head Boy and Head Girl, but is it for each house or only two for the whole school?
Thanks for the information.