First off, it created lots and lots of new tests, most of which weren't properly validated before being sent out there. Also, good tests had already existed. When I went to school in NY, from grades 2-8, every year we took what were called Achievement Tests, which seemed to do a pretty good job of assessing where each student was. It was three days towards the spring of the year, we got the results before school let out, and there was no special prep for them. Just the regular curriculum all along.
I also had the experience of sending my sons to a very good, academically intensive, secular private school, and I was amazed at how much more content they taught than the very good public school they'd been going to. As late as the day before the last day of school they'd be learning new material in their classes. None of the turning in of textbooks a week before the end and watch videos that last week which did occur in the public school. If you teach plenty of content, and have various ways to check on a child's progress, you don't need to give them very much test prep for one simple round of tests. And a student who scores significantly worse than his performance on a day-to-day basis is a special case, someone who probably needs just a little instruction in that kind of test taking.
It's not rocket science, but it has created huge inequities above and beyond those that already existed.
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