Why has the modern educational establishment become so obsessed with standardized testing and uniformity of measurement? When did so many people start believing that you could plot learning as numbers and graphs — and still teach children what they need to know?

That’s what I kept thinking as I read about a $1.1 million project to look into designing “galvanic skin response” bracelets for students to wear that would measure their degree of engagement with what they were listening to. The goal is to find out whether physiological feedback from these sensors could tell teachers whether students are learning or not.

If you’re not familiar with the phrase “galvanic skin response,” it’s just the name for the process of measuring how electricity flows through the skin depending on changes in moisture. It’s the principle upon which so-called lie detectors work. (Those things are bogus, but that’s another story entirely.)

Is this really the way some people believe we should decide whether kids are learning? Are we going to keep going until we strap a sensor helmet on kids every morning so we can monitor their thoughts for the full day?

Part of the problem is that some people want to turn teaching into a science. They want to make everything quantitative, so they can make graphs with numbers that allow them to compare teachers and schools and cities. There’s nothing wrong with wanting to evaluate teachers and schools, but the obsession with testing and measurement is the wrong way to go about it.

If you hang around teachers long, it doesn’t take too long to figure out which are the good ones and which are the bad ones (and which are mediocre). It was traditionally a principal’s job to know which teachers are worth having on staff and which should be moved out the door. If the principal is competent, he or she can evaluate teachers and make the changes that are necessary. So why do we end up with various ridiculous schemes to tell us which teachers are useful and which aren’t?

It’s because of the twin horrors of tenure and lawsuits. In most states, if a teacher is in a school system for a certain amount of time — generally three years — he or she has tenure, which means it’s almost impossible to fire that teacher. Why do we have tenure? It’s supposed to protect teachers from political pressure since schools are run by the state. At least, that’s what we’re told.

It’s getting harder and harder to fire bad government employees — thanks in part to unions — but the clerks down at your City Hall or county government don’t have tenure. If they’re not doing their jobs, they can be fired. If a bad teacher makes it three years and gains tenure, you’re stuck with him. Why are teachers so special that they get what amounts to lifetime employment in government-run schools?

We really need to get rid of government-operated schools and let the market supply education chosen by parents. But since we’re currently stuck with government schools, we need to at least get rid of tenure and let principals go back to being held accountable for how schools perform. If a school is lousy today, the principal can honestly say his or her hands were tied. If there’s no tenure — and the principal has the power — that person should be held accountable for the performance of the school he or she runs.

If you really think that galvanic skin response is a good measure of how well students are learning, you don’t know teachers very well. The good teachers I know can tell me what their kids are learning and what they’re not learning — and how to modify what they’re doing to reach them. There’s no test that can measure the things that really happen in a teacher’s head that tell him or her how to do what’s right for each student. What’s more, the very best teachers care deeply about their students — and that’s something that the educational establishment’s tests can’t measure, either.

This particular bracelet is allegedly supposed to be a tool to help measure student engagement. I guarantee that whatever their tests show, a good teacher already knows whether that student is engaged in learning. Forget about the fancy galvanic skin response measurements. They really don’t matter.

Let teachers do their jobs. Let principals do their jobs in hiring and retaining the right teachers. Then let’s hold those principals (and superintendents) responsible for performance. If they can’t do the job, replace them with people who can. It works in every other business. Why shouldn’t it work in education?