NEW YORK--(BUSINESS WIRE)--Fitch Ratings has affirmed the senior and subordinate notes for SLM Student Loan Trust 2012-6 (SLM 2012-6) at 'AAAsf' and 'A+sf', respectively. The Rating Outlook remains Stable for the class A-2 notes and the Outlook has been revised to Negative from Stable for the class A-3 and class B notes. The Outlook revision is due to Fitch's belief that the class A-3 and class B notes both carry a heightened level of extension risk. Based on Fitch's cash flow modelling runs, t