Cho is suposed to be a year older than Harry, but she’s in the Room of Requirement before the Battle of Hogwarts with the other students. Harry should have been in his final year (if he’d still been at Hogwarts), so shouldn’t Cho have finished the previous year?
From the book:
"Harry’s mouth fell open. Right behind Lee Jordan came Harry’s old girlfriend, Cho Chang. She smiled at him.
“I got the message,” she said, holding up her own fake Galleon, and she walked over to sit beside Michael Corner."
They were all (Ginny, Fred, George and Lee Jordan) coming in through the tunnel to Aberforth’s.
Cho may have actually been in the same year as Harry.
Why do we assume that Cho was a year ahead of Harry? Probably because such is indicated a couple of times in the series. In Prisoner of Azkaban we have:
"Harry, I've just found out who Ravenclaw is playing as Seeker. It's Cho Chang. She's a fourth year and she's pretty good....
So Oliver Wood explicitly tells Harry that Cho is a fourth year. But how reliable is Wood? He's a seventh year who has nothing to do with younger students in another house. It would be quite simple for him to mix up a third year as a fourth year.
In Goblet of Fire, when considering dates for the Yule Ball, Harry thinks:
Cho was a year older than he was; she was very pretty; she was a very good Quidditch player, and she was very popular.
But is Harry any more reliable than Wood? For all we know he is only basing this on the facts that Wood told him a year earlier!
On the other, we have a statement from Cho herself that strongly implies that she was in the same year as Harry. In Order of the Phoenix when they go on their date we find:
"Remember the first time we played against each other, in the third year?" she asked him.
Note the words "in the third year". This clearly implies that they were both in third year. If she had actually been in her fourth year she should have said "in your third year", or "in my fourth year", or "when you were in third year and I was in fourth year", or "two years ago".
Thus, it is possible that Harry and Cho were actually in the same year. It is not surprising that Harry did not realize this. At this point, it seems that Harry had never had a class with Ravenclaw, so he wouldn't have been with Cho even if she was in his year.
Note: in any case, as mentioned in the other answer, it is clear that Cho only arrived at Hogwarts for the battle and was not there before. But she might have left school for any number of reasons.