I read somewhere on this forum of a parent saying that their child had done well in the CAT test (whatever "CAT" means??) and that their score of "99s" was good. Yet somewhere else I saw that one should anticipate 130 for Pates level.
An average score is not bad as such, and I would be surprised if any teacher said "your child is average" at a parents evening. They may well say "your child has done really well" and mean "for what your child is capable of, they've done really well - I was expecting them to achieve a below average score, and they've actually managed an average - brilliant (for them)".
And that is how parents are mislead into believing that their child is of grammar standard, when in fact they're just average, but could do quite well in a good comprehensive, and be stretched to the top of their ability.
Of course, if it was 99%, then that's an entirely different matter!