No this would be difficult to work indeed
I mean what I say. The whole setting and the way in which the books have developed since the first (with alot of fun, new and interesting things that got people reading the series) would leave an RPG gasping for things to do. I am not personally a big fan of the series - it is fun to read and watch on film but I never consider it to be the most defined or elegant creation.
But I do like RPGs so here are my thoughts:
-- I think that setting it when HP is a young boy is definitely the best thing to do. That way there is no interaction with the infamous characters that appear later on. Obviously people know of HP and what happened, but he is being brought up somewhere else as far as they are concerned.
-- Geographically, stick with Hogwarts. In fact if they do venture beyond it (in the holidays for instance) then stay in Britain (sorry America, but we know nothing of the wizarding world elsewhere really...). There is plenty to play with there anyway.
-- As far as adventures and time goes, the thing to do is make each term an adventure as it were. Something during that term needs investigating/solving/doing. The first year would maybe be an introduction and simply learning things and interacting with everyone would be the primary goal (wow a real rpg eh?). Later on things might occur that need sorting out. One thing must be done however, so that the characters don't assume they are like HP and fling themselves into danger hoping to survive. If necessary have them rescued and then severely punished.
-- Characters themselves would work well on the D20 Modern system I think. Levels would be gained near the end of each term (or in the holiday) if well planned. XP awarded for gaining house points, completing tests, earning prestige etc... I would not give out XP for defeating things in combat (which would be mostly avoided) but enhance the roleplaying by rewarding acting in character. I say stick with one generic class - keep it simple. When you go up a level you can gain a new feat/ability and more skills (representing your terms learning). Races would be either magical family (default), muggle family (penalties in certain interactions and less interesting holidays, but probably a talent to compensate) or just plain muggle (only abilities that can be gained are non-magical). I guess you could even play a muggle, if you thought of a good reason to be there! Perhaps you are one of the teacher's relations?
-- Magic system based on skills would be the easiest to do. Not sure how many or what they would be but this is all just thought. Feats would allow things above the norm (such as a skill focus - bonus to a skill, or talent - max ranks slightly higher) and special abilities such as influence or allies. Not sure on this again - but it would be nice to incorporate the possibilities of quidditch teams, extra-curricular activites and prefectdom into the feat system. Also houses - based on highest stats as mentioned above I think. House feats sound a great idea. Magic itself would be a certain amount of power/day (obviously used during class, alas) skill checks to use. Can only attempt spells you know (failure can lead to fun affects of course. Other skills would be generic ones.
As I see it a HP type character would be the charismatic, lucky sort - high charisma with feats to represent his luck. No spectacular skills other than those he learns normally. A Hermione type would obviously be from a muggle family and be talented because of that. She would have some real life skills, and of course be better out of Hogwarts to a certain extent. Ron would be something like a sturdy sensible type... lots of allies in his family.
Anyway someone phone up JKR and ask for permission and we could write something

!