There are hundreds ways that your school community can raise funds to help pay some of the excursion costs for your students. Students can either individually sell chocolates etc, with all funds raised going to their own excursion payment, or your class can organise fundraising events with the total funds raised being divided between all of the excursion participants.
The fundraising directory site provides access to hundreds of companies, sorted by category and by state. The fundraising resources section of their site gives rules and regulations for each state, for conducting various fundraising activities.