You may or may not need to go to school to become a comic book/manga artist, so here's an example of one in Oregon.
Portland state university
Portland State University is located in Portland, Oregon. It includes a comics studies program, where you would probably want to go if you wanted to create comics that were similar to manga. Comics Studies at Portland State University is a vibrant, dynamic program on the cutting edge of contemporary literature and art practice that serves it's students, it's mission as a university, and it's local community.
There is no degree required in order to get this job. Although you may need to be a good artist and/or writer, it doesn't really have any special requirements, and everyone has their own art style.
Typically, comic book/manga artists are paid by how much work that they get done, not necessarily the amount of time that is spent on it. So it would be real helpful to have a crew work on your book with you rather than trying to get everything done yourself.