It is unfair to reward teachers for results they can’t influence  The success of a student depends on many factors, like innate talent, the ability for hard work and concentration and socio-economic background. This means that any progress that a student can make is largely outside of a teacher's control. This will result in some teachers being rewarded just because they happen to teach in a good environment to 'advantaged' children whereas other teachers who do a good job in a bad environment to 'disadvantaged' children are just unlucky.