I can see it from both sides. Its really more the principal of the matter in this case.
You could put a notice out that as of this date all field trip fees must be paid for by the first of the month or something like that then further it by saying failure to pay will result in the your child having to miss out on the field trip. Some parents just dont get it! There is no easy answer I guess. It is just a dollar here and there.
I bet all it would take it one time of there child having to stay back and they will understand. Good luck.
Deb

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