As KarenG says, ask the specific questions. As what quartile your child is in, and what level in the National Curriculum your child is at, or expected to be at by the end of the academic year.
I've had essentially the same problem with my son's school. You try to get an answer out of them and you just get wooly statements about him being "at or above the level expected". After bashing away at it for a couple of years I think they realised that I wasn't going to accept that as an answer, so, on request, they would tell me where he was performing on the NC levels.
What helped was that I was also armed with the information they have to make the assessments, the "National Curriculum level descriptions for subjects". This document seems hard to find, but there's a PDF here:
http://www.fft.org.uk/FFT/media/fft/Eve ... ptions.pdf
As it turns out, he is just finishing year two, and they expect him to attain level 4a – apart from anything else, I think this just shows how low the government sets the attainment levels (given that level four is the average at year 6).