Accommodations should be included with the student’s test registration, and if there is a problem with that, one would find that out when they contacted the school to make sure everything is ready to go. I agree if for some reason accommodations aren’t part of the student’s registration, the school would not allow the student to have those accommodations…not to mention they might not even offer those accommodations anyway.
Test proctors will have nothing to do with making sure there are appropriate accommodation settings for every student who is registered to take the test and approved for accommodations…that typically falls to the school’s testing coordinator (if there is one), a college counselor (if there is one), or a social emotional counselor.
ETA: here’s what CB says about test accommodations:
Before Test Day
Families should check with the school’s SSD coordinator to make sure accommodations are in place and no further documentation is required—especially if the student was approved at another school or the family did not go through the school to request accommodations. If a student is approved for school-based accommodations, it is important to notify the SSD coordinator of the dates that you intend to test.
On Test Day
On test day, all students testing with accommodations should remember to bring their SSD eligibility letter.