There are two very damaging old chestnuts about education.
First chestnut: "A school should be run like a business."
A school is not a business, it's a school.
If you have a disruptive employee in a business, someone who acts and acts out against the best interests of his/her own self and of other persons (employees), that troublesome employee can be terminated.
The profit motive of a business demands it.
If you have a disruptive student in a classroom, someone who acts and acts out against the best interests of his/her own self and of other persons (classmates), that troublesome student can NOT be terminated. Instead, it is demanded that the teacher somehow deal with that situation while at the same time, teaching everyone to some high standard.
There is no profit motive in a school.
Second chestnut: "You can't control what you can't measure."
In a business, you can measure productivity. You can tally up how many new accounts did this or that salesman land this year, how many new products got developed this year, how much profit did the business make this year and so forth. With plenty of measurements, you can guide your business decisions and at least have some chance of being in control.
In a school however, if you seek to measure a teacher's performance with a tally of class grades, the numbers you get can be a very poor measure of the teacher's "merit".
When my wife was teaching High School Art, I often saw her assigned to two classes in the same subject, Oil Painting for example, with one class getting very high grades and making all kinds of marvelous achievements including many students receiving "Golden Key" awards for their paintings while the other class got lower grades and where every lesson became a struggle for control of the room. This was with having the same course materials, the same curriculum, the same lessons, the same daily schedule, the same classroom resources, the same teacher and yet there could be two widely differing sets of results.
By which class might one have made a judgment of merit?
Similar variability can also be seen from term to term and from year to year. So, why is that so??
The answer is that no matter how skilled, how well trained, how perceptive, how astute, how insightful, how competent any teacher is, the teacher's traits or "merit" are only part of what yields student performance results. With a classroom of all cooperative students, results will very likely be sterling, but get just one difficult student in there, one student who won't shut-up during a lecture, one student who won't stay in his/her seat and there will be some detrimental effect on the entire student body of that class. Get several like that and the classroom situation can very easily be dragged down to mediocrity.
That part of the situation will be reflected in test scores, but those scores have nothing in them that can be separated out by analysis as a reliable gauge of teacher "merit".
