"The issuers—student-loan packagers like Navient Corp. and Nelnet Inc.—do not want their bonds downgraded. The investors who own the bonds do not want them downgraded....You just extend the maturity date. If the bond matures in 2026, and you aren’t sure the underlying loans will be repaid by then, you can just extend the maturity to, say, 2055."
https://www.bloomberg.com/opinion/articles/2020-01-07/no-rush-repaying-these-student-loans