Lack of work in class

As others have said, the assessment to classwork contrast indicates an issue with busy class environments, definitely something I remember having a lot in school. Classes suck if I'm honest. They're always noisy, they're always distracting, and they make work really difficult. How is his homework writing? That may give an indication of lower stakes but calmer environment work?

If it is the classroom environment then this is some stuff I suggest looking into

You may want to discuss with teachers having the ability to leave for quiet spaces if things get too much. Library maybe? or they may have a specialised room or set of rooms for this purpose, I suggest talking to your SEND team about it. 

Outside of leaving the class here are some of what I considered essential for most of my school career:

Some kind of noise reducer, I was undiagnosed at school but like your son I scored extremely highly on assessments so staff had no basis on which to prevent me from using earphones, ear defenders, ear plugs and headphones unless they were unreasonable despite it being massively against the typical behaviour policy. Some staff are unreasonable, you may or may not have to push for this. I think for me, I wore them so openly, all of my teachers assumed someone else had given me permission. And as I said, I performed better this way so there was no evidence for them to tell me to take them off. 

Another really important thing for me was the ability to avoid or reduce group work. I was an incredibly unsociable child, particularly in class. When class sizes allowed it (which they mostly didn't until a-level) I would sit in a corner with a clear seat to as many sides of me as possible (usually ended up being by the teachers desk) and if someone was sitting next to me I wouldn't talk unless addressed and often not then. Many teachers did allow me the option of working alone, some did not, again this may be something you have to push for. 

An additional accommodation to think about, is access to written versions of the lesson. For maths this was often a textbook (coupled with the fact I had a special interest in maths so was often working on something else to the rest of the class). I had a very tech based school, iPads were considered core equipment and every student was provided with one, so most teachers lessons were whiteboard based. By GCSE and for some subjects earlier, pretty much every teacher of mine sent me the PowerPoint before class started, or early in class once prompted, so that I could work through it at my own pace. It is quite possible this may be a difficult thing to get, especially if, as it seems, they have decided to label his struggles as a deficit in learning behaviour. It may be your child learns from videos better than reading, there are videos on certainly every maths topic up to a-level and many for the other subjects too, particularly at GCSE. 

I hope some of this helps and your son gets the support he needs to thrive. 

As others have said, the assessment to classwork contrast indicates an issue with busy class environments, definitely something I remember having a lot in school. Classes suck if I'm honest. They're always noisy, they're always distracting, and they make work really difficult. How is his homework writing? That may give an indication of lower stakes but calmer environment work?

If it is the classroom environment then this is some stuff I suggest looking into

You may want to discuss with teachers having the ability to leave for quiet spaces if things get too much. Library maybe? or they may have a specialised room or set of rooms for this purpose, I suggest talking to your SEND team about it. 

Outside of leaving the class here are some of what I considered essential for most of my school career:

Some kind of noise reducer, I was undiagnosed at school but like your son I scored extremely highly on assessments so staff had no basis on which to prevent me from using earphones, ear defenders, ear plugs and headphones unless they were unreasonable despite it being massively against the typical behaviour policy. Some staff are unreasonable, you may or may not have to push for this. I think for me, I wore them so openly, all of my teachers assumed someone else had given me permission. And as I said, I performed better this way so there was no evidence for them to tell me to take them off. 

Another really important thing for me was the ability to avoid or reduce group work. I was an incredibly unsociable child, particularly in class. When class sizes allowed it (which they mostly didn't until a-level) I would sit in a corner with a clear seat to as many sides of me as possible (usually ended up being by the teachers desk) and if someone was sitting next to me I wouldn't talk unless addressed and often not then. Many teachers did allow me the option of working alone, some did not, again this may be something you have to push for. 

An additional accommodation to think about, is access to written versions of the lesson. For maths this was often a textbook (coupled with the fact I had a special interest in maths so was often working on something else to the rest of the class). I had a very tech based school, iPads were considered core equipment and every student was provided with one, so most teachers lessons were whiteboard based. By GCSE and for some subjects earlier, pretty much every teacher of mine sent me the PowerPoint before class started, or early in class once prompted, so that I could work through it at my own pace. It is quite possible this may be a difficult thing to get, especially if, as it seems, they have decided to label his struggles as a deficit in learning behaviour. It may be your child learns from videos better than reading, there are videos on certainly every maths topic up to a-level and many for the other subjects too, particularly at GCSE. 

I hope some of this helps and your son gets the support he needs to thrive.