Over the years I've published some lists and reviews of free tools for creating videos online. Quite a few of those tools have been for creating simple videos that are really just automated, audio slideshows. See Animoto for an example of this. There's nothing inherently wrong with having your students use those tools, but at some point you will want to take your video projects to the next level. These are the five tools that I recommend for creating and editing videos without installing any special software.
Pixorial
WeVideo
PowToon
Wideo
Weavly
Via Gumersindo Fernández