That's terrible. I'm sorry you're having to deal with that. I teach theater classes - acting, musical theater, playwriting, etc. for kids in K-12th grades. It is a program run through our local university and there are fees for the classes, so the kids are only there if they really want to be, which I know makes a huge difference!
Last year I took 13 of my high school group on a 7 night trip to New York. These are kids who eat, sleep, and breathe theater. We flew from Arkansas, spent 7 nights in a hotel, saw 5 Broadway shows, did acting classes, musical theater classes, dance classes, special backstage tours, discussion sesions with actors and directors, etc. It was a really huge deal for the kids - absolutely the trip of a lifetime. Total cost was $2,300 per kid. They did raise some of that (about $300 each) themselves with a few fundraisers, but the parents had to pay the rest. We had payment contracts and they knew that each monthly payment had to be made on time, or they would be canceled out of the trip and loose the money they had already paid.
This year, I'm taking 11 of my junior high and high school students to Disney World for 8 nights. We're doing a special acting class at Epcot, a video production class at MGM, a backstage tour of the Cirque theater before we see the show, discussion sessions with actors and designers, and lots more special stuff. Total cost is $1,700. Again, we had payment contracts and they knew that each monthly payment had to be made on time, or they would be canceled out of the trip and loose the money they had already paid.
Happily, I can say that we have had no issues at all (money related or otherwise). Of course, I know there is a big difference between my situation and a public school situation. Like I said, the kids involved with these trips have devoted a huge amount of their lives to theater. Most of them have been in our programs since they were very young and they are planning on lives in the theater. So, they are thrilled to have these experiences and they behave accordingly.
I can tell you for sure, though, that if they were behaving the way your students are they would be dropped from the trip immediately and wouldn't be invited on any future trips! (Next summer's trip is to London!) Of course, you may not have that option in your situation.
I'm really sorry this is ruining what could be a fabulous trip for you and the kids!