She comes to Hogwarts a year ahead of Harry Potter. She is Sorted into Gryffindor and plays Chaser on the House Quidditch team.
Since Katie is a year older than Harry, she would have been born between September 1, 1978, and August 31, 1979. She is mentioned as being a seventh year when Harry is in his sixth:
They returned to the common room, which was empty apart from a half dozen seventh years, including Katie Bell, the only remaining member of the original Gryffindor Quidditch team that Harry had joined in his first year (HBP9).