Do driving instructors fail a certain amount people on purpose?

Driving instructors do not fail anyone, their job is to teach people how to drive. It is Driving Test Examiners that fail people. Examiners do not have a set quota of people to fail they observe the student doing a standard set of maneuvers and mark the students performance for each of these objectively. A student will need to obtain a fixed score for these maneuvers to pass the exam.