A.N. Ginny is the same year as everyone otherwise it wouldn't work.
No one would have thought that Hermione Granger (Know-it-all Gryffindor Muggle-born), Draco Malfoy (Slytherin Pure-blood, son of a death eater), Harry Potter (Boy-Who-Lived Gryffindor), Ronald "Ron" Weasley (Gryffindor Player, Weasel), Ginerva "Ginny" Weasley (Weaselette), and Annastasia "Anna" Karma (Slytherin Know-it-all Pure-blood) would all be friends by the end of their time at Hogwarts. Hell, not even Professor Trawleny knew.
Here's how it started actually, back in their first year.
A.N. I know it's short, it's only 69 words. But hey, it's the prologue. It's allowed to be short.
Mysteries are meant to be solved.