Right before Mother's Day, people suddenly realize that they haven't gotten something nice for their mothers. Instead of letting them run out to the local drugstore, encourage them to buy some flowers, along with a card that says the money has been donated to the school in honor of the mother who receives them. You can order the flowers directly from a nursery, and people can pick up the flowers on the day itself. This is especially helpful to parents who carpool, as they can save themselves a trip to get a Mother's Day present. You may want to include a less expensive option for students who want to surprise their mothers as well.
Garage sales are a great way to make money with very few expenses. Parents are often happy to provide their kids with boxes of old toys, books, and other products that will sell easily, if for nothing else than to get these items out of the house. The school library might sell some of its old books as well. It does take a lot of time to organize all of the products, label them by price, and display them in a way that allows browsers to find what they're looking for. In addition, you won't be able to charge much for each item at a garage sale. At the same time, you won't be paying anything to buy the items themselves, so any extra money you make can go straight towards the school.
School carnivals are a lot of fun for students and parents alike, and students will often line up to volunteer for them. If you're raising money for a wide age range of students, you'll need to find activities that can be scaffolded to fit that range. For example, at a ring toss booth, preschoolers can stand very close to the pegs in order to toss the rings, whereas high schoolers and adults will need to stand farther back. You can also arrange to have a student talent show to help convince families to show up to see their children perform. Carnivals cost only as much as supplies for each booth cost, although you'll have to decide whether it's worth it to invest in some of the bigger and more expensive activities, such as a moon bounce, which is an inflatable trampoline-like box, or a dunking booth, in which children can throw a ball at a target to make a staff member fall into a small pool.