What is some good advice for talking to a 13 year old who is losing interest in school? The main reason this is happening is because she really is trying hard to get A's but always seems to fall short. She's frustrated.
The first thing I would do is discuss why she feels such a strong need to get As. Does she feel less worthy when she gets a lower grade? Is she worried about college? There is nothing wrong with getting Bs and even Cs, when you try your best.
It sounds like she is putting too much pressure on herself. What you learn is far more important than the actual letter grade, although I understand the drive to have a high GPA. If it's really bothering her a lot, perhaps she would be open to tutoring (either in the subject or general learning strategies) to push her up a bit.