I have used this tool to do the comic task. I liked it a lot because of the amount of scenes, characters and prompts you can choose from to create the story. Indeed, I have already created a Halloween story to work with students.
PIXTON: The most powerful and easy to use comic and storyboard creator I've been able to find. It is a bit limitted in characters and costumes but you can change their pose and expression...You can play with them like with puppets!!!