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Dropping IQ at that age isn't the same thing as "losing abilities". So a child who can perform simple arithmetic in 2nd grade but can no longer do that in 4th grade is a bigger concern than a child advancing, but doing so at a slower rate than their peers.
IQ in children is tricky. If he tested at 87 when he was 5, and then at 60 at the age of 10, he may well not have lost any actual functioning intelligence, he just hasn't advanced as far as his peer average has during that time.
IQ is measured by mental "age" over chronological age in children. He may actually be advancing in his abilities, but since he's not advancing as far as his peers, his IQ number compared to them (and that's what IQ is, it's a ratio number, with average being calculated at 100) is dropping.
It's not so much as to if they administered the right test as to how they administered the test. If it was a timed test - its not valid. Any child with your sons difficulties should be administered an untimed test - even give days to finish it. Furthermore, if they want a true idea of his capabilities, they probably should have somebody there to read or answer questions or at least de-stress him. So yes, you should (depending on how the test was administrated) take it with a grain of salt.
And ya, your IQ can drop (in a way). IQ tests are based on a standard set of information that a person should learn year to year. If they don't learn as much each year (due to things like ADHD), then you would expect to see a drop in IQ. Its always interesting to look at the sub sets of the test. For him, I would not be surprised to see the one part still rising, but the other part falling.
I can understand his teacher wanting to increase his reading and math goals. I question if it can effectively be done. Or in other words, how is it being done. If he is being helped one on one, this could be very effective (although this depends on how it is being done - rote drill would probably not be good). If he is just being given a bunch of papers to do. Then he would be much better off going to classes where he has an interest.
And, I do worry about the "he can't handle them' phrase. He can't do the work - or he causes disruptions due to the different environment or the other teacher not being able to deal with him?
I know that giving stim meds to kids who have seizures is a very mixed bag. Is he on any thing for the ADHD? And, if you don't mind, what meds is he on for his seizures?
Finally, I always recommend the book, "The ADD/ ADHD Answer book," by Susan Ashley. Its probably worth picking up (or buying via amazon), as it will help with his ADHD problem.
Hope some of this helps. You have a huge responsibility, and I feel for you. Best wishes!!
I was just reading the other day that IQ can change. It has ups and downs and it isn't set in stone. He's still young and has the potential to change his situation. If you are concerned about the results is there anyone you could take them to? Do you have your own therapist and doctor that could read them and possibly retest? You have great questions and it seems like you are really thinking things through. You have to be an advocate for your child and make sure that his wants and needs are expressed to everyone who is involved in his care. I'm not an expert on this, so I hope you do consult one who can really help you! There are a couple other people on this forum who know a lot about ADHD who may have some valuable input too.