By the time you've gotten to this point, the reasons should be no surprise to the school, because you've had a number of meetings in which you've expressed your concerns, to anyone who would listen.
We've withdrawn our DD8 twice from the same school she's currently attending. She doesn't have any disabilities, but we do live in a state that legally treats the needs of gifted children the same as the needs of any child with exceptionalities. We have found it to be useful to be frank but factual about the reasons our DD has been withdrawn, because the next time she shows up, those reasons are still valid, and there needs to be a plan to address those concerns as a precondition for her re-enrollment.
The principal now avoids us like the plague (yet purposefully gives a fake-friendly greeting to my DD whenever she sees her, which is creeping DD out), but we haven't damaged any other relationships to the point where they're unworkable, and so far my interactions with the new asst. principal have been fairly positive.
As in your case, we withdrew our DD based on the school's failure to respect her rights as guaranteed under the law. I've found it to be fairly helpful to frame it as such. So in your case, if anyone asked why I was removing your DS, I'd say the following, as if I were a bored court clerk reading out charges to the accused: "I am withdrawing my DS because he has a physical disability that has been diagnosed by a medical professional, and the school has failed to offer appropriate accommodations for his disability, as required under the Individuals with Disabilities Education Act."
