It's just a measure of the fact that you can find a lot of people who are willing to do that job

From The Merits of Merit Pay by Megan McArdle. She is addressing a common argument that confuses the perceived moral value of a job versus the market value of a job. It is economics 101 but not commonly understood and she puts it well.

We do, in fact, pay teachers pretty well relative to their level of education and hours worked. But that's a side point. The real point I wanted to make is that you hear this sort of thing all the time, and it misses a fundamental point about markets: it treats them as if the demand-side were all that mattered. In this mental model, the supply of workers is fixed, so the only thing that matters is how much we want them; the more we want them, the higher we bid their wages.

But of course, there is another side to the market: supply. And the reason that teachers are not paid as well, as, say, chartered financial analysts is that it is considerably easier to meet the requirements for being a teacher than to pass the CFA exam. It's no accident that the professions which require a lot of math pay more than the professions that are based more around reading and writing. Nor that math and science are the hardest positions to staff at most high schools.

But the majority of positions in high schools, or primary schools, are not staffed by math and science majors. They're staffed by humanities or education majors, who are relatively plentiful, and also, in less demand elsewhere in the market. With a glut of candidates, schools don't have to pay that well, and they don't.

This is not a measure of their value as people, nor of their value to society, nor even of whether society recognizes that value. It's just a measure of the fact that you can find a lot of people who are willing to do that job.