ToonDoo is a great site for creating comics online. Kids can create their own accounts and choose to keep their work private, or make it public. ToonDoo has a lot of built in characters, and has a tool for creating your own from scratch. You can also import characters and backgrounds that you find elsewhere or create in a paint program such as Paint.NET. ToonDoo allows the user to choose from different comic layouts, and to combine several comic strips into a book.
Your students can create lots of different kinds of comic strips.
Here are some examples:
Hope, the Girl of Life - Original Story
Harry Potter - Parody
Thunderman Prologue - Thunderman's backstory
Jokes
I want candy - Adventure story
Superkids - Superheros (find the Easter eggs!!)