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.
School admins are the only ones who can assign a teacher to a class, and assign pupils to a class. They can do so by logging into their 2Lasso Administrative Dashboard and clicking on the class or staff. See instructions for doing so here.
A staff can be assigned to multiple classes. This is helpful for school admins, who may want to see pupil work or help with assignments (2Dos). This is also useful for part-time staff or teaching assistants who may float from class to class to help pupils. School admins may wish to assign those staff to any classes where they regularly assist pupils with their Purple Mash work.
The same principle applies to Groups. Teachers have to be assigned to Groups in order to work with pupils in a group.