The concept of for profit school obviously is recent compared to the idea of writing a novel so the class of novelists who didn't go to novelist school obviously contains everything before the student loan boom.
Stephan King, former English teacher and janitor.
Issac Asimov, former biochemist
Umberto Eco, former philosophy degree followed by TV/Radio editor
J.K. Rowling, former BA in French who preferred to read English Lit and worked as a secretary, clinically depressed and unemployed
Hunter S Thompson, former high school dropout, journalist with some audited journalism classes
Ernest Hemingway, former high school grad, served in WWI, journalist with no higher education.
I'm sure there's someone out there with best sellers or Nobel prizes AND went to a writing school, but an anecdote or two proves little.