Harry and Ron did not return to Hogwarts after Voldemort's defeat. Hermione did and she completed her N.E.W.T.s.
SU: Oh, (MA:Well, did...) speaking of Ron and Hermione-
JKR: Yeah, did they graduate from Hogwarts?
SU: Yes. Did they?
JKR: Harry and Ron didn't go back. Hermione did.
SU: (gasps) Ooo!
JKR: Did you bet right? I mean, come on, nobody's going to think Hermione wouldn't go back.
SU: I predicted. Yeah.
JKR: Of course she'd go back. She has to get her N.E.W.T.s. Ron was really done with schooling (laughs). I think that (SU: Yeah.) it would be kind of tempting to go back just to mess around for a year and have a break, but he goes (JN: Yeah.) into the Auror Department. He's needed. Anyone who was in that battle the right side. Kingsley would want them to help clean up the- I mean anyone who's old enough to do it, who's over age (JN: Hm.), but Kinglsey would have wanted Ron, Neville, Harry, and they would've all gone, and they would've all done the job. And I think that would've been a good thing for them too, because to go through that battle, and them be relegated to the sidelines, I think they would've felt a need to keep going, and finish the job. So that would've have been rounding up the corrupt people who were doing a Lucius Malfoy, and trying to pretend that they really weren't involved.
[J.K. Rowling Interview with The Leaky Cauldron - 12.23.07]
And there is the author's word :)