They can log in with their SSO credentials on their school page.
Single sign-on users will be directed to their school page after they choose their school on the application.
This same process applies to schools using Google Single Sign-On. If your school has set up one set of login credentials to use across many apps, the school can assist you with login or reset your password if needed.
In general, it's best to contact your instructor for assistance logging into Schoology.