According to PowerSchool's release notes for ver. 22.214.171.124 released 11/16/2020 and rolled out to hosted customers on 11/21/2020, the mobile app single-sign-on issue has been resolved. I was notified by PowerSchool Support yesterday evening (presumably because this thread illuminates their lack of care for paying customers) that the issue was resolved and I needed to refer to the release notes; rather than sticking to their promise of keeping me apprised of developments and changes.
We tested our student sign-ins this morning and were able to successfully sign in for the first time since JULY!
Here's a screenshot and link for the referenced 126.96.36.199 release notes.
Aside from being updated by support well after-the-fact, I'm glad our 1000+ student population can finally use a product we pay for.
The issue that was experienced in JesseB3's district has been resolved. If you are continuing to experience an error message, you'll want to work with the Administrators at your school or district to look into the server being accessible and confirm the mobile app is enabled for users. The Administrators will also be able to review your account settings and work with PowerSchool Support for any further troubleshooting.