GoAnimate - Cartoon and Animation Creator
The neat thing about GoAnimate is that you are not creating a comic strip, but you are creating a cartoon. With GoAnimate you can create your own character, or select one of the many already created for you. You can select from a wide variety of backgrounds as well as artifacts. Since it is animated you can select from a number of actions for all your characters as well as emotions. You can also move your characters from one scene to another without any interuption. The most amazing thing is how easy it is to learn how to use the software to create the cartoon. See the video below for a short tutorial.
Pixton - Comic Book Page Creator
Pixton is less detailed in terms of options than GoAnimate, but it still offers tons of options for creating your comic book type page. You have the ability choose your characters, backgounds, artifacts, and different thought/word bubbles. For quicker projects, this would be a more ideal software to use. Although Pixton does allow animation, it is not near as detailed as GoAnimate.
Toondoo - Comic Strip Creation
Toondoo I think is the best Comic Strip Creation tool. You create a comic strip just like you see in the Sunday paper. It provides you with a lot of the characters and backgrounds, you provide the text. Several students at my school have used them. They created some as part of an assignment on parables. You can view them here. I highly suggest you visit and play around with the site before showing it to your students. It is very user friendly as well. Students will need to create an account though.
Other Comic Strip Creation Websites