Time for a question! In the fourth book there was mentioned that cho is one of the fith year girls, but then suddendly in DH she is back at Hogwarts. What do you think happened? Did she came back to help or failed her examens?

It did state in the book that some students had returned. Maybe Cho was one of them.

Since she was a member of Dumbledore's Army back in Order of the Phoenix, she could have been summoned for the fight as long as she'd kept her Galleon. That's how all the older students who'd left Hogwarts by that year knew to return (other than Oliver Wood, who'd left before OOTP, so someone from Gryffindor must have contacted him).