Right now, Teachers are unable to add a student to observe without a workaround. The problem with this is the workaround would allow students to observe each other (and that's not a good solution to share openly as it could lead to cheating).
Teachers who elect to combine their Canvas roles into one account would benefit from having the "Observing" button within the Account Settings. How do I link a student to my user account as an observer? A teacher who does not add a student from their district/institution in this space would have an uninterrupted experience.