I would be talking to someone at the District Office about this "policy". It is not fair for a child to be left behind just because her/his parents can not arrange to go on the field trip with them. I know you are planning on taking her to
Disneyland anyway but I am sure she would enjoy the trip with the other students and teacher so much more this time. The next day the kids are going to be talking about what they did, say, etc and your little one is going to be left out of that also. Again, I would be knocking that door down demanding that my child not be excluded from the interaction of a field trip with her peers!!! Kids can be cruel and you can bet that those little ones will know that she was not "allowed" to go.... Better to put a stop to this now instead of letting it go on any longer. Good luck!