If you have only one instructor in your diary then all pupils are allocated to this instructor automatically and this feature is ignored by the software. Otherwise this feature is very helpful to filter those pupils that the selected instructor is working with.

You can set this filter in the Filter List

Press F4 key to open Filter List and Select Pupils Bar. If Pupils filter is hidden, click on its bar to open it.

Click the button on the right to drop the list and set: Pupils attached to instructor = Yes