A lot of how you set up fundraisers for school trips depends on your monetary goals and how many kids will be going on the trip. For smaller, more local trips, or smaller classes or clubs, it may be enough just to have the kids do a small direct sale fundraiser as a group effort. The good thing with direct sale fundraisers for school trips (such as candy bars) is that you know ahead of time much money you can make, because you buy a certain amount of product up front, and you already know how much profit can be made from that amount of product. If you get the kids all working together on the project, then these kinds of fundraisers for school trips can be just right for more moderate goals.
If the trip is longer or farther away, fundraisers for school trips can become more complicated, because the amount needed is is greater. It may be more useful to think of things in terms of the individual kids, and how much each one needs to raise to pay for their own trip. This can actually be a huge motivational factor - if the kids know that they are raising money on a personal basis, and that their efforts with fundraisers for school trips will directly affect their ability to go on the trip, often they will work harder at reaching the goal. On the other hand, you don't want anyone to panic if their fundraiser effort isn't going quite as well as the others, so it can be a good idea to have a couple of extra adults working on the school trip fundraiser in order to help out.