In the books, it's clearly written that Cho is one year ahead of Harry academically, but in Deathly Hallows we find Cho in the Room of Requirements with the other students. Given the fact that she should have graduated the previous year, what was she doing there?
More specifically, how did she manage to get into Hogwarts despite Snape being headmaster?
In the books, it is clear that Cho Chang arrived when the Dumbledore Army called everyone with the fake galleon.
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.
Rowling, J.K.. Harry Potter and the Deathly Hallows (p. 474). Pottermore Publishing. Edición de Kindle.
For example, Fred and George were there too.
