Chogger lets you easily create a comic, with no registration required. What’s particularly nice about it is that you have a choice of drawing it or searching the Web for images you can insert.
I’m adding it to The Best Ways To Make Comic Strips Online.
I learned about it through Jennifer Stern and Joyce Kasman Valenza, who have just published their own list of comic-creators.
Considering the sometimes “exquisite” (but unfortunately not uncommon) Terms of Use – http://chogger.com/terms – I cannot really recommend using this in the classroom. The Google image search alone may produce nasty copyright problems, as image sources are not listed with the finished comic. A Creative Commons filter, or similar, does not seem to be in place. – Maybe the Classroom version of Chogger – http://classroomcomics.com/ (not available yet) – will offer a less worrying framework.