Now I understand that not all kids have the same abilities
or backgrounds, and both make a big difference in how far
those kids can go. That needs to be managed, but it needs to
be managed in the standardized process.
If our society were prepared to manage the fundamental issues that plague our educational system, then your rationale makes perfect sense, assuming that the standardized lesson plans were designed to facilitate the assessment of meaningful programs. But we're not prepared. Rather, the standardization efforts were an excuse to avoid addressing the fundamental issues, and have become a distraction that causes authorities to focus on the wrong aspects of educational reform.
The overwhelming factor in educational achievement of a population is the quality of home life, and in particular the participation of adult family members. Poor families have neither the time, money, nor enculturation to facilitate meaningful improvements in home life. Without a social commitment targeting the root causes, such as by providing free day care, the standardization efforts achieve nothing, and unnecessarily constrain local control.
Since the Bush-era standardization reforms, the achievement gap has only _grown_. And we're still matriculating kids without the requisite skills as the alternatives, like holding kids back year-after-year, are neither desirable nor practical.