Using comics to teach isn't just for those who have difficulty reading and writing, but it is great for those individuals too! I think, without exception, everyone I know who is really into comics are also rabid readers. There is something about comic book nerds that makes the printed page irresistible to them. There are so many uses for comic books, like teaching Onomatopoeia. Science fiction has been a mainstay in comics for decades. But you can learn science from them as well. Star Trek inspired many an invention, but these are actual comics about actual science. Or how about a Unit Study for the younger kids - to cover everything from Math to Art. Speaking of Art, keep some of these handy by printing off some blank comic panels for the kids to create their own. Not so much into drawing? You can still make your own at Make Beliefs Comix. Turn literature into art, older kids can do a book report of their Literature assignment by making it into a comic strip.
In short chances are if you want to teach it, there is a comic out there that covers it.
Not sure which comics to start with? Here is a fairly comprehensive list from the Graphic Classroom.