As Karina said you need to focus on what they can provide that your current school can't. If she is good at some sports which they are big on put that, if they offer a language she wants to do etc.
Another thing they look at it ease of getting to the new school, transport availabilty etc. Also how will it fit her social needs better?
What can she bring to the school? Has she been chosen for debating teams? sports teams? any awards?
I will chat to some high school teacher friends and see what they advise since I am only coming from a primary perspective.