Maybe going back for her 8th year was a mistake. She knows all the material, after all – she'd be able to take her N.E.W.T."s without going back, if she wanted – but she didn't want to leave her education unfinished. More often than not, though, her teachers are covering things she'd already learned and mastered during the Horcrux hunt, and she's left to her own devices to learn anything new.