Harry feels he has bigger fish to fry this next year, but in order to be an Aurer he'll need to complete his 7th year. How do you suppose Rowling will reconcile that?
Do you suppose that by doing all he needs to do to get the remaining horcrux's and killing Voldemort, he would "qualify" to be an auror?
Besides, if it's the last book, JKR could just leave us hanging on that! LOL!
I think the members of the OOTP and especially the Weasleys will talk him into returning. Will he stay there all year? I don't know.
What makes everyone think that the next book has to coincide with the Hogwart's school year? It might start from the day after DD's death, for all we know.
You can pack a lot of action into a few weeks -- certainly enough for a full length book. Harry might have everything wrapped up neat and tidy before summer vacation is over.