iMovie is the ideal tool for those working in a 1:1 iPad or Mac environment because it allows a great amount of customization without requiring bandwidth. The iPad app is extremely easy to use, even for elementary school users. Both the iPad & Mac apps come with preloaded trailer templates or the ability to create projects from scratch.
WeVideo is the ideal solution for Chromebook users and anyone not in a 1:1 environment. It works much like iMovie in functionality and ability. It features preloaded Creative Commons media and integrates well with a Google Drive account. WeVideo for Schools offers a lot of great extra features. Check out this chart for a side-by-side comparison.
Creative Commons Search is the ideal launching point for finding Creative Commons licensed media to support video projects. From photos to video to background music and sound effects, Creative Commons Search is a great starting point for finding supplemental media.