Teachers are responsible for facilitating the learning and development of each of the pupils in their care. They provide a rich environment where students can absorb not only knowledge, but also practical lessons they will use throughout their life. The expected level of knowledge increases as students move from grade to grade, and a student's success is determined on how well they are able to process this knowledge. Educational standards ensure that only well-prepared students are able to advance to the next grade. When a child does not meet these standards, whether through apathy or inability, it is not only appropriate, but necessary that teachers allow them to fail.
Failing teaches students that there are consequences for their actions. When students are habitually truant or do not complete their schoolwork, they have not met the minimum expectations, and thus should not deserve a passing grade. In fact, a passing grade would be tantamount to awarding poor behavior. Students are smart, and some will figure out how to abuse the system by doing the bare minimum of work. Teachers must be able to discourage this behavior by doling out failing grades. Only then will these poor performers learn discipline and the importance of cause and effect.
Failing a student may help place him in a group of his intellectual peers. When students cannot stay on track with the teacher or cannot learn the required material, it may be that they do not have the maturity or mental faculties required to be successful in that grade. Education is entirely progressive. When a student does not know the fundamentals, they cannot hope to learn anything complex. Permitting them to advance would only overwhelm them with information they do not understand, and leave them behind. In such cases, it is a teacher's responsibility to recognize their students' shortcomings and do them the service of failing them. Then, remedial teaching can correct any deficiencies and allow the education process to continue.