A single student can sometimes totally change the dynamics of a class--for better or worse. When it is for the better, you may not even take note. When it is for the worse, it can stop you in your tracks. Usually, however, it is two or more students, playing off of each other, who possess the power to totally shift the dynamics of a class.
When supporters of VAM concoct their formulas to measure acceptable growth for given subsets of the student population, they fail to recognize (or adequately account for) so many factors. I would argue one of the most important factors influencing student learning is not so much the teacher as the dynamics of the class.
Although a qualified and experienced teacher may have better luck in dealing with and influencing the dynamics of the class, her power is limited. Unlike the new teacher, the veteran has survived. She may have a reputation most students will not wish to test. Yet, even the best teachers in some schools can end up with students who will not listen to reason. They cannot focus. The problem may be compounded by the time of day.
The classroom may harbor students (and sometimes it only takes one) who cannot be controlled by even their own parents or guardians. The class may be perpetually on edge, only a moment from a potential eruption. Despite the talents of the teacher, "effectiveness" may take a dive toward developing during this period. Put that in your formula as you evaluate teachers who are asked to teach to someone else's test.
Take a "highly-effective teacher" from a privileged school or a privileged program and put her in a room in which classroom management means dealing with people who don't care to hear you. There may be one or there may be thirty-four of them. Good luck. Watch her struggle. Watch so much of her expertise go down the drain. How much is "highly effective" really worth then?
Take an inexperienced teacher and give her an obedient body of students. Provide a setting, as in some charter schools, in which disruptive students may be summarily dismissed. Watch her shine with her review book in hand--while, perhaps, her most troublesome student suffers in a padded room somewhere. He is so far removed, he can't even disturb a living soul with his crying. Let us hope this changes some day.
In truly public schools, there is a commitment to educate even those who would be a thorn in the side of the teacher. Charter schools, at least some of the more newsworthy, seem to pull out their thorns and stick them in the side of the truly public schools. These charters can then sigh with relief and, in their next breath, gloat about how their test scores put the public schools to shame.
When I stop to contemplate the potential power of a few kids over the academic outcomes of education and the future of teaching, it is not really so surprising. After all, doesn't a small group of very well endowed, yet inexperienced "reformers" have the potential to "creatively disrupt" the entire educational system that is the very bedrock of our democracy? Sometimes the most harm is done by the fewest people.