In a lesson, if an exercise has twenty questions, you will usually get through five to seven representative ones. The teacher then sets the rest as work.
The second thing I ask of students is exactly that: finish the paper. Twenty questions means all twenty.
Why a class cannot cover everything
Not because time ran out by accident, but because class time has a better use.
My classes run on CELTA principles, which means student talking time should be around eighty percent. Most of the session is learners speaking, not sitting and listening to answers being read out.
Across those ninety minutes you practise not one sentence but a great many. Three hours a week, split over two sessions.
Spending that time reading out answers to twenty questions would be a waste, because reading answers is something you can do perfectly well alone.
On mock tests
People pay to book a mock test with a teacher.
Think about what you are buying. What differs from practising alone is the marking and the feedback.
The practising part you can do yourself. The prompts exist, model answers exist, and the tools to record yourself and compare exist.
Paying for the part you cannot do alone makes sense. Paying for the part you can is worth a second thought.