Select multiple groups and multiple pupils
By default, the All group is selected, showing all the existing pupils in the pupils list. The user can select a specific group by clicking on it, it will show all the pupils inside this group in the pupils list. The user can also select multiple groups by clicking on the checkbox on the left of the groups name, it will then show all the pupils inside all the selected groups in the pupils list. The All group is a special one that doesn't have a checkbox (as it's not needed for it as it always shows all the pupils), and also doesn't have an edit button to rename or delete it.
The user can select one pupil by clicking on its line. The user can select multiple pupils by clicking on the checkbox on the left of the pupils name. There is at the top a "master" checkbox to select/deselect all the pupils displayed in the list. This way it is easy to select either all pupils, or all pupils from a single group, or all pupils from a set of selected groups.
Selecting pupils is used for the actions Add to group, Remove from group and Remove pupils, as explained in the next points.