Here's an idea that will generate some competition between classes, homerooms or Sunday school classes as students work together to reach a common goal (raising funds for your organization).
The basics of a penny fundraiser are very simple: every class has a jar to collect pennies in, with each penny standing for a single vote for that class.
To add interest to the event and attract more funds, let any other coin denomination act as a vote against the class — a nickel equals five votes against, for example. The jars should be placed in a public location so everyone has the opportunity to cast their votes.
At the end of each day empty the jars and keep a running total. To give everyone chance to bring in their pennies, keep the event going for at least a week — you could have it running during school spirit week perhaps.
You might want to publicize the running totals as you go or keep them secret until the end depending on what works best. Announce the final result during a pep rally when the fundraiser is over. It's fun and easy to organize, and can raise more money than you might think.