Sorry I wrote my email right before I saw your latest post, OP.
Yes, you should protest. You can respond and cc the cluster supervisor and the compliance office explaining that you have volunteered to act as a chaperone.
What district, OP?
http://www.montgomeryschoolsmd.org/departments/special-education/contact-us/#school-based
"Refusing to allow a student with a disability to attend a field trip may be a denial of a Free and Appropriate Public Education (FAPE) under the Individuals with Disabilities Education Act (IDEA), as well as Section 504 of the Rehabilitation Act. Prior to excluding a student with a disability from a field trip, the school must consider if accommodations and supports will allow the student to attend, thereby giving the student equal access to the school program."