Ok guys so I go to an old British Grammar School, which is probably close to how Hogwarts is run, so here's our system.
We have to elect a council from our year group, there are four housein our school and 1form equalls one house - so anyone in K form is in one house, anyone in M form is in another house etc. In the Upper Sixth/year 13(UK)/or 7th year (HP) anyone who wants to be considered for head boy or head girl in that form puts their name forward, and then when all of the nominees are submitted, the form votes on who they want in their shortlist. then these lists get sent to the Heads of houses who collectively - with the headteacher choose one boy and one girl from each form who become the heads of houses. Lower down the school, in 5th Year (HP) the forms vote for form prefects - 3 boys and 3 girls from each form and these prefects remain prefects in their 6th Year.
So put really simply :
5th Year Prefects - 3boys 3girls per house - voted in by peer group
6th Year Prefects - 5th Year prefects retain their prefect status into the 6th Year
7th Year Prefects (Head Boy and Girl) - 1 boy 1 girl per house - shortlisted by peer group and voted for by Teachers
und dann später auf Nachfrage:
In 7th year the HB and HG can be picked from any student in the year - not just the prefects. In my school, (in each of the 4 Houses) we have 12 prefects from 5th and 6th year collectively and a HB and HG of that house, giving 14 prefects in total from each house.
Und mugglenet.com äußert sich ganz ähnlich:
Prefects/ Head Boy and Girl
The appointment to be Head Boy, Head Girl, or a prefect was a great honor (well, except for Fred and George Weasley).
Head Boy and Girl.
At the beginning of the school year the headmaster appointed one Head Boy and one Head Girl from all of the seventh-year students. Although this was likely to be a student who was already a prefect, it wasn't always the case (James Potter was Head Boy, and we know he wasn't a fifth-year prefect). The Head Boy and Head Girl are in charge of the prefects and should be an example to the other students (although why anyone would want to be like Percy Weasley is a mystery).
Tja, und damit wäre Rileys ursprüngliche Frage beantwortet - James konnte durchaus Head Boy werden, ohne vorher Prefect gewesen zu sein.
Der Übersetzungsmythos ist damit noch nicht geklärt, aber wer weiß, vielleicht bekommen wir das auch noch heraus.