I cannot really give you any pointers on online schools. Frankly, I think a great deal of them out there are scams so be careful and do your research.
I do know that The Joe Kubert School that I graduated from offers correspondence courses. You purchase a kit that gives you a lesson plan, you draw it out and send it back and one of the instructors will critique it and send it back. There are several levels. The school is all about comic book illustration and cartooning.
Here is there website
http://www.kubertschool.edu/