Well, attending the field trip means being with the kids the entire time. Our 504 said the same thing and I was always on the bus. The field trip starts when the kids get on the bus at school and ends when the trip returns to school. When you sign the permission slip, ours lists the field trip as being from ____ time to ____ time (such as 8:30 A.M. to 2:30 P.M.) and that defines the field trip as starting when the bus leaves school. So IMO the school is wrong by saying you cannot ride the bus. They are violating your child`s 504. I would find out which your child prefers---not is he okay with you following, but rather which does he prefer. I personally don`t take well to 504 violations. Is your ds at the age where he specifically does not want you there? That did not start for us until middle school. If he prefers you go, I would insist that they follow the 504. I don`t think the 504 needs to spell out that you are allowed to ride the bus. If it says you attend all field trips, then it is obvious that includes each part of the field trip. I think most members here with 504s have it in there that we can go on the field trips, and I don`t think any of our 504s actually specify that we are allowed to ride the bus. I think most people assume attending the field trip means all parts.
I know what I would do. I would write a short note to get the paper trail going. I would write something to the teacher in charge saying that per the 504 you will be attending the field trip from the time the kids leave school until they return. BTW, I had to call our Office of Educational Equity Compliance over something that relates to field trips, and I am positive that they interpret it to mean that the parent rides the bus. I am in California.