Is that average over the country? Because there are regions where almost every student is already above average, and regions where they are below.
This sounds like a way of funneling the best teachers into the already best schools in the richest areas, and making it impossible to employ good teachers in poorer regions -- unless you have a plan to fix this (I realise you've only given a tiny explanation).
This would seem to involve independant, standardised testing to be held on all students, every year. Letting teachers, or even the school, evaluate students would obviously make it too easy for biases.
I will admit I'm in the UK, not the US, but we only do huge standardised tests every 3 years or so, as they are very disruptive.
This sounds like a way of funneling the best teachers into the already best schools in the richest areas, and making it impossible to employ good teachers in poorer regions -- unless you have a plan to fix this (I realise you've only given a tiny explanation).