Animoto is super simple to use! Start with still images, add music from Animoto's choices (bunches!) or add any music you want using an MP3 file. Teachers can apply for education licenses here. These come with 50 free student accounts. When you have students use accounts created in this way, they get more features than they would if they signed themselves up for the standard free account (which is limited to only 30 seconds per video).
See a student created book trailer here. This is a great book trailer, and features uploaded images from LetterJames (an online image generator).
Here's a student produced promotional video for Windsor County, done as part of the 7th grade's interdisciplinary units.
Here are some book trailers and other short movies that were created for library or school promotional use.