Dude, I don't know how Hogwarts, let alone the UK put peoples in grades, but where I'm from it goes by the year, and you stay with the other kids in your year... well unless you get left-back or brought up a grade... But you should do it whichever way you want, if i was reading your fic i wouldnt have noticed a thing, or cared to check
Good luck with your fic, the idea may be over used but i still LOVE it!