This is an example of a trip form that we use when we go on a Museum trip downtown Toronto.
Obviously, it can be tweaked to fit your own trips. All the parts in red are what need to be changed - really easy.
It also covers the specific word'ing for students to be on their own, unsupervised, during the lunch period.
It includes an itinerary as well, to help with parents understanding.
The second form is what parents have to sign (it's a printout that is provided by our school board, so therefore it's not included here)
I hope this helps!
NOTE - it's formatted to print on your school letterhead as well..it just may need a bit of tweaking.