I agree with Startail. My daughter did the CAT test in Yr 6 in an independent school. The first we knew of a problem was when we were told that she would not be able to cope with an academic secondary. Trouble is that the school suggested would have required her to take three buses, and she would probably have been the only one of her year even applying.
A quick survey of her teachers at parents evening suggested that aside from English (she is mildly dyslexic) they all thought she was doing fine and keeping up with her peers. They had not spotted any problems. The English teacher had not been briefed that she had been assessed and found to have a few problems, but once she knew, was brilliant.
As a parent there seemed something wrong in deciding that though your child is doing well in school with a number of nice and reasonably bright friends, you make a decision, based on a test whose results you have not seen, that your child is not academic and should go to a different school. I would feel the same way if the issue came up over triple science. If a child is doing well despite poor CAT results surely they should be given credit for achieving over expectations.
The whole process was painful and ended up with us saying we would only apply for schools that our daughter wanted to go for, even if none were on the school's recommended list. (Luckily the school went up to 13 so she had a fall back.) At one point we were threatened with a poor school recommendation.
It turned out fine. She got two of the five, and was waitlisted for a couple of others. We then paid for an independent test (expensive) which confirmed that she was very one sided causing her average to be low, and effectively meaningless. We were told not to worry. Once she got to sixth form she would be able to choose her subjects and should make a strong University candidate.
In practice two years into her Secondary she is doing fine, indeed she seems to be considered one of the brighter girls in what is a pretty selective school. She is absolutely happy.
CATs may be useful in suggesting children who may be under-performing, but it is very sad if they are being used as a secret 11+ to determine future academic pathways.
In Starfish's position I might try to find a sensible member of staff (form tutor, head of year, or perhaps the science teacher) to talk through the issues and how the process works. If individual teachers agree that your child is doing fine they might be willing to put the case forward for keeping options open.
(Sorry long post, but using CATs alone to determine setting targets, setting etc is a strange and worrying process. )