If teachers have permission to join or manage their classes, there is a possibility that they could add a teacher, or an administrator to their class. You will want to make sure that their OneDrives are not visible from the Dashboard.
The easiest way to protect all staff members' OneDrives from accidentally becoming visible is to ensure that anyone who is not a student has (at least) the role of 'Teacher'. You can do this on an individual user basis or in bulk.
To bulk assign the role of 'Teacher' you should add all non-student users to a single group within Teacher Dashboard or Azure Active Directory (if you are using the TD, AAD sync feature).
Go to the groups page and find the group with all of your teachers. Select the checkbox next to the group name and pick 'Assign role(s) to members'.
In the popup window, select 'Teacher' and click on 'Assign role(s)'.
Now, even if a teacher is added as a student to a class, their OneDrive will not be visible.