A great online site for students to go to make animations is Aniboom.
When the student has finished they are able to export the file as an animated gif to use in PowerPoint presentations, web pages etc.
However there is a need to be careful here as there is an area where you are able to watch animations that others have made and some have adult content.
Another program that is available - free to download is Scratch.
They have excellent support with pdf files and videos on the site to help teachers and students. I have uploaded the Getting started with Scratch pdf file into the files on the left of this site so you can download that and get an idea of the program.