ETA
I worked a LOT of years with kids with disabilities.
Here's the fact. IF IF IF he has a disability he may NEVER get this material at the same rate as other kids. IF IF IF he has a disability should they keep him in first grade forever because he can't get that material? Of course not.
If he has a disability then they school should have already had the school psychologist in the room observing him and already provided you with the names of places you can take him for eval.
IF IF IF he has a cognitive disability they should not flunk him because of it. They should provide him with an IEP that covers this. If he is only low but not significantly under average then they should provide him with pull out programs to work on those specific topics he's having trouble in.
Kids with disabilities do NOT get flunked because they don't know the material. They get pulled out of class to work on reading, math, and more. The school bends over backwards to help this child be successful.
They DO not perpetually flunk him because he's disabled and cannot grasp the concepts and "get" it the same as the other kids. THE SCHOOL has to step up and do as much as they can.
How do you think kids with Down's Syndrome graduate high school and they can only write their name and do a tiny bit of basic reading? The school cannot flunk them due to them not grasping the information. The kids are mainstreamed and they work with them as best as they can but they progress to the next grade just like the other kids.
You should have the testing/eval done now. This week. He deserves to have as much going for him as possible.
You mentioned learning disabilities. He is not like other kids. He is different and nothing anyone does can fix a disability. It is a life long part of him.
He may NEVER get the material for first grade. legally they can flunk him 3 times in each grade. So, next March, when he still doesn't have the material down are you going to be wondering if he should do first grade again?
Get the evals done so you know what you're working with. A disabled kid is different and they don't have to fit the mold. He deserves to have every advantage he is legally obligated to have from that school.
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I'd say put the older kid in summer school and make them pass him. If he's at all able to do the work the summer might give him the advantage and get him passed.
I hate when kids ae flunked. I know it has to happen sometimes but it's so detrimental for the rest of their lives. They always have to explain why they're a certain age and in "that" grade instead of the grade they're supposed to be in.
My grandson was failed in first grade due to his dad bringing him 5 minutes late to school every day. He was counted off a half day each time. Dad got off work in another town at 7:30am, picked up kiddo from the overnight babysitter in that other town. He was ready to go to school. Took him straight to school but he got there at 8:15 in stead of 8:10. They often hadn't even done the morning announcements yet.
It was really sad. They said legally they couldn't pass him. Dad couldn't just quit work, this job paid really well. So kiddo failed first grade. He knew all the material and was actually ahead of the rest of the class. His teacher often gave him extra work to do and he excelled at all he was given.
He shot up over 6' in 5th grade. He was so humiliated to be taller than even the majority of the teachers. If he had been in the next grade up at the Jr. High he would have fit in just fine. They all had their almost adult size growth spurts that year.
So try to get him passed. Try everything you can.