Gonna post and run, because I have other work to do, but here goes:

Around 2009, I was considering a change from software. I applied to both Teach for America (TfA) and the Boston Teacher Residency (BTR. Similar, but at least in principle involves more training and you end with a masters degree in education) and got in to both. I applied to teach high school math.

Brief digression: Why that route and not following the traditional route of college? Because I wasn't willing to sink a shit-load of my own money into taking a (massive) pay cut. Digression over.

I went to BTR and got my first firsthand view of how the education system works from the inside. The comments below are principally about the education system, not BTR.

shantly's comment [0] is spot on. There is a huge emphasis on testing, even in a school that is trying to emphasize broader learning. The school I was placed at had pretty minimal classes outside of the same shit we all suffer through. I believe the only music class available was choir. That may have been the only art class at all.

The math teachers I worked didn't ONCE meet with teachers of any other subject to do any discussion let alone planning about to alight subject matter. There are some pretty obvious ties between most math and most science classes. With a little effort, I'm pretty convinced you could find ties to non-STEM classes too, but everybody is focused on their own silo.

Outside the building, it's worth remembering that the standards/"best practices" change with a frequency that virtually ensures that every student will live through a shift in methodology. Remember No Child Left Behind? Me either; I was out of school for that, but whatever that was gave way to the seismic shift of Common Core some time in the late aughts/early teens.

Nobody (within rounding error) spends a lot of time reworking their entire curriculum/lesson plans every time a shift is made, because everybody knows that it's going to be changed again soon enough. In due time, the better teachers will have updated piecemeal as they have time and see what parts of the new methodology are good. They'll be done just about in time for the next big wave of change. Students, meanwhile get caught in the middle of these changes as everybody is trying to get their feet back under them.

If it wasn't bad enough that the politicians feel the need to change things up regularly, you have the well known problems of the morally bankrupt textbook and calculator industries. Unless of course, your school doesn't have enough money to have a book for every student (not a hypothetical; I never saw a math textbook during my time in BTR).

But wait, it gets worse: is your boss unskilled in your area of practice? Don't worry, if you're a teacher, you're almost guaranteed to be in that situation. Two types of teachers try to get into administration. A few good, idealistic ones, and a lot of crappy ones who don't much like teaching and would rather appreciate a pay raise.

The problem propagates upwards. At some point you leave the bureaucratic side of administration and enter the political side. Good luck finding a skilled teacher there. Unfortunately, since everybody has had the experience of going through school, a lot of people have strong opinions of how school should be done. Periodically, one of those people gets high enough to drive a change, and you end up with the aforementioned seismic shifts.

And then the press comes around a couple years after a shift and writes a whole bunch of articles along the lines of "We fixed it: Why isn't it better yet?". The answer is because your pipeline extends almost literally to the birth of the student. But nobody wants to face those problems and everybody wants a quick fix.

Full disclosure: I decided that while I didn't much want to be in software, I didn't much want to be a teacher either around February. I would have been lousy anyway, at least at first: I'm not much for multi-tasking, and I didn't want to spend several years trying to get good and letting my programming skills get out of date. All for the aforementioned big pay cut.

[0] https://news.ycombinator.com/item?id=21692026

[1] Most teachers would rather appreciate a pay raise, but most of them aren't interested in going admin to get it.