Alright! So for this year since there's not many seventh years anyway, it will be first come, first serve for Head Boy/Girl.
Next year, however, anyone can nominate their character to be Head Boy/Girl providing they are suitable.
So this is for the current sixth years. The voting will occur when August in rp comes around. Just because such and such characters were canonically Head Boy/Girl, doesn't mean they will be here. It's AU and therefore anything can happen. You don't need to currently be a Prefect, but they need to be reasonably outgoing, compassionate and with leadership (or potential leadership) qualities, though not in the dictator kind of way. Of course, if you think differently, do say.
Do bear in mind their workload and responsibilities, so they're not biting off more than they can chew.
Head Boy nominations:
Head Girl nominations: