You can make your teacher's eligible for subbing into certain (or all) classes from their teacher profile. Click Studio Setup > Teachers > click on the a teacher.
To the right of 'Auto-substitution eligibility', click the dropdown carrot to expand the menu.
If the class is listed here, the teacher is eligible.
Assign any additional classes they need to eligible for
Use the checkboxes to remove classes.
If you see a messages stating, "You don't have any upcoming classes yet," or "No classes found" it means that all upcoming classes have been assigned to this teacher.