It depends on how the test is given in my opinion. If it is not a timed test, and if the tester works to develop a relationship with the child and continually refocuses the child - then the test can be fairly accurate (even more so if its an oral test). If it is not done this way, then I would doubt the accuracy of the score.
If the teacher has had the time to get to know your child. Has had the opportunity to talk to him one on one - then she should be able to give you a pretty good feeling as to how he compares to other children in the class "intellectual" wise.
I have found that as a teacher, I could not use the child's test scores or homework to get a adequate feeling for their ability as there ADHD got in the way. Particularly, at a younger age.
It was only through one on one conversations or even group discussions, that I could get a real feeling for their abilities.
If a child scores high on such a test then there is no doubt that he is intelligent. If he scores poorly this can be for a number of reasons. The most obvious is mental retardation, but there are other factors. I am reminded of the old IQ test. In the 30's and 40's northerners scored an average of 110 and southerners 90. Since there was no intelligence difference between the two populations, then the test itself was deficient. I know that there has been progress in devising such tests, but do they take into account the focus of the person taking it?
You sound like an observant, objective mother. If you feel there are no intellectual deficiencies, then we must assume aren't any. That means there must be some rethinking in his treatment.
It sounds to me like the School staff is telling you what is wrong with your child. Please what ever you do don't let them just lable your Child . You need to have your child tested by Doctors and make sure that they look at everything . Meaning blood work ,eeg to make sure his brain is not having Seizures that you don't know about because there are so many kind of Seizures so seeing a Noroligest would be great .
Try given him Vitamins like omega 3-6-9 and B6 ,B12 and a childs multivitamin they can all help better than those drugs they use for for people that they say have add or adhd and bipolar.