> High school math is not a rapidly changing field

Are you sure? Here is a list of 16,000+ papers written about teaching high school math since 2018: https://scholar.google.com/scholar?as_ylo=2018&q=pedagogy+up...

Also, as a teacher, you have to adopt the district standards, which change every few years. Certain areas are removed, others added, some things are more important, some less. Those standards are changing to meet new standardized testing requirements.

There is a lot of change happening. Just like in programming. People who learned Cobol still need to learn new languages once in a while, because things change, even though the principles stay the same. It's the same data structures, same algorithms, but yet software engineering is rapidly changing.

I always phrase it as the principles (mostly add/delete) stay the same, but the methods are almost always different. Once you learn the principle, it's always about the method of getting there.

That's still work though. You might know all you need to know on the subject you will teach, but if the standard you have to fulfill changed, that means you have to change your plans.

Most of this change is driven by something other than need. The high school mathematics curriculum is a bit of a funny beast in the US, but it is certainly not the weak point in the system.

The desired goal of high school math may not change much from year to year, but the best strategies for meeting that goal certainly do. I cannot imagine asking a student today to learn algebra the way I did 55+ years ago. Log-tables and slide rules? Probably not a good strategy.

Until one has taught something that is completely new to a student, it is difficult to imagine how challenging that can be, and how individual it is. What helps Johnny understand (or even care) is often completely different from what helps Mary. It is very difficult to teach effectively without figuring out what the student does not understand. And there are as many ways to misunderstand as there are students.

That is why teachers keep revising. They want to make the material more accessible to more of their students.

The material may stay the same but the reason why a child may not understand it will vary.

In addition to changing cohorts that others mention - What are the chances the way you taught it last year is the most effective way?

Good teachers experiment - maybe something they taught last time didn't go over too well, how can that be improved? Can they make the material even more relevant this year?

Also, it makes teaching it more interesting, rather than regurgitating lessons. Teachers are human after all.

If kids were learning high school math just fine 50 years ago (and, as far as I know, they were), then that suggests that advances in pedagogy are either not forthcoming, entirely irrelevant, or overwhelmed by other factors.

It doesn't "suggest" that at all. Perhaps changes in pedagogy were necessary in order to adapt to a changing world.

Education isn't like the naive "encoding/decoding" model of communication, where the subject matter is simply "transmitted" from teacher to students. Even if the subject matter remains stable over time, many other things do not: changes in the media of communication, signal interference (say, from the average classroom size drastically increasing over time), all kinds of changes in the teachers and students themselves, changes in society's expectations of what constitutes success or failure (e.g. rote learning is now widely seen as having many shortcomings), changes in what students actually need to move forward (a career in the trades may well require a much lower standard of understanding in the age of the pocket calculator), ... this list could go on and on. Teaching is not a "solved problem" like that.

50 years ago, high school math at most schools ended with trigonometry. Today a large fraction of college bound seniors have taken calculus, and STEM students have taken two years of calculus. And yet, 50 years ago there were lots of math-phobic kids who, today, are expected to perform at some modest level (50 years ago, they stopped with algebra I).

It is true that 50 years ago a fraction of kids were learning just fine, but more recently the goal has been to make that fraction larger, in a society that actively devalues learning.

Your day is 8 hours long and includes 5 hours of meetings, 1 hour which is composed of duties and and maybe 30 to 45 mins for lunch. You've now got 1.5 hours to make "small adjustments" to the 4 classes you teach. Also, maybe you need to do some grading, deal with unruly kids, document what you did for those kids with IEPs, field emails, and adjustments for whatever latest fad the school admin is applying to the curriculum.

The material is not so static. Finding square roots by hand is no longer taught as part of the curriculum.¹ Interpolation from data tables is also not typically taught since students are expected to have a calculator that can at least handle trig functions and logarithms. Graphing calculators enable solving problems as part of classwork that weren’t possible before the calculator. When I was in high school, the lowest-level math class offered was Basic Math which didn’t even cover fractions. Some schools (but not the one I attended) topped out with AP Calculus BC which was roughly equivalent to a Calc II college class (integration and series). Now, the lowest-level math class available at the high school level is Algebra and students cannot graduate without passing Algebra and Geometry.² Some schools offer AP Calculus CD which covers multivariate calc (e.g., Calc III) as well as AP Statistics, neither of which existed when I was in high school.

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1. I’m sure that there are occasional classrooms where this is still taught, but it’s more an enrichment topic than part of the curriculum.

2. I was in high school when Illinois raised its graduation requirement from one year of math to two years of math. The year after that happened, my high school began offering Basic Math 3/4 in addition to the Basic Math 1/2 class it had previously because too many kids were unable to do fractions.