MK is quite right -- if you have to sign a permission slip for him to attend, then obviously it isn't a "must". That's what permission slips are about!

Our school has assignments associated with at least some field trips -- DS11 went on the 7th grade field trip to the dam when he was in 5th grade (because he was taking classes in several grades and wormed his way into all the field trips) and then this past year when he was actually IN 7th grade, he chose instead to go to his track meet that day and missed it, and he had a zero for the assignment of writing up water test results. But if the grade for a possible assignment doesn't bother you or him, I see no reason he "has" to go.

DD freaked out this past year about the 2nd grade trip to the science museum, but I knew it was just fear of the unknown and made her go anyway -- of course, as I knew she would, she liked it.

Stick to your guns, or if it's not a battle worth fighting up front, do as someone else suggested and sign the slip and then keep him home anyway. What can they do?