phrase.it will let you add cartoon word bubbles to your own images. I have all kinds of ideas about how you might use this for school projects--is your mind running yet? At it's simplest, you could have them very easily create a comic. More complex: I'm imagining multigenre lab reports in science class including a comic-style series of pictures to illustrate their procedures.
No sign-in required, so students could use it with out an email account. Completely free, and very, very simple to use, and free and simple is the best!
Do take a minute to read the fine print. You have to own the images you use, so keep that in mind. Also, be wary of how this sort of tool could be abused. (Have you met middle schoolers?) But with some guidance, this could be a great, useful tech tool to add to your toolbox.