I do feel that it should be based on the term thus far, and how responsible the students are being. If it were two or more assignments, it would make more sense. Everyone messes up sometimes due to tons of circumstances. I think behavior should have a bigger restriction, because their behaviors are what will make or break the trip. If I forgot one assignment, that doesn't mean I'm going to be a cut up and an annoyance on a trip.
Field trips are essential to the learning process.
It's a silly idea. You need to pick an appropriate place that is either very specific to a subject being studied, or successfully intergrates the long term projects of several classes. This will help the students identify the point of the trip and see that there is something to be learned. Also, there has to be a product. I don't mean set the kids loose in a museum with a worksheet to complete, but you've got to have a point to going and discuss it with the students before and after the trip.