Why a teacher is unable to see their pupils or classes
All staff, including school administrators of a school's Purple Mash license, must be assigned to a class to see pupils. If a teacher is not able to see their own pupils, it is because they are not assigned to a class.
In other words, a Purple Mash staff login only allows a staff member to login and use Purple Mash themselves. It does not automatically know which teachers are assigned to which classes, which pupils are in which classes, and so forth.
The same principle applies to Groups. Teachers have to be assigned to Groups in order to work with pupils in a group.