Earlier this week, the Consumer Financial Protection Bureau issued a final rule that allows the bureau to supervise for the first time the nonbank servicers of private and federal student loans that qualify as “larger participants” in the student-loan servicing market. With an emphasis on supervision, the rule is not expected to have much of an initial effect upon the secondary market for student loans. But the CFPB’s expanding role into the sector could change that, especially if there is a crisis in student-loan lending. The bureau’s new rule defines...