Teachers aren't paid to personalize lessons though, if you work overtime to do that then you can only blame yourself. Teachers are paid to hold standardized lessons and answer questions help students with problems.
You could argue it would be better if teachers were paid to personalize lectures, but they aren't. Maybe some document somewhere says they should, but in practice nothing will happen to you if you just use standardized lectures so that is what most will do and that is what the expectations of the job is built around.
You say this because you don't know better, but its very common in schools for kids with learning or other disabilities to be in the same classes as other students - called 'mainstreaming' https://en.wikipedia.org/wiki/Mainstreaming_(education)
Teachers are 100% supposed to teach the standardized kids, the kids with disabilities, and the kids who are a little faster than everyone else. It's simply not possible to do with one standardized lecture without individual attention to those with special needs.
It's not some document, its literally a major movement decades old in education.
Lesson plans are not standardized down to the day and order.
Sometimes, a group of students might need to speed up, slow down, or go in a wholly different direction depending on what happened with the previous lesson. There’s a lot of variation between one class of students and another, and teachers are not some kind of omniscient perfect predictor of behavior. And ultimately, you need to cover a whole curriculum.
There is a whole spectrum between teaching centrally dictated lesson plans and individually crafted tutoring, and one of the requirements of teaching is to be able to adjust lesson plans according to reality. We don’t expect software engineers to waterfall every last character in code down to the wire, but teachers are supposed to do this with unpredictable human children?
You could argue it would be better if teachers were paid to personalize lectures, but they aren't. Maybe some document somewhere says they should, but in practice nothing will happen to you if you just use standardized lectures so that is what most will do and that is what the expectations of the job is built around.