You can check my answer below. But I am myself not convinced that is the best method.

The 'Before' and 'After' system can work well for coaching. But good teaching is not necessarily coaching students to produce better results in tests. According to me, a good teacher will help students gain an interest in the subject, appreciate the subject better which in-turn helps you perform better. A student is capable of flunking in-spite of being taught well.

In my opinion teachers should be assessed by parents and the management by having a system where every now and then parents are allowed to be part of the class.