But there are things that can be done to mitigate it.
So in Texas schools get so many late-arrival days. This year my district was allowed 3. When most kids were testing, the seniors and juniors showed up late. I teach srs and jrs, and I was a proctor for the test.
The test coordinator arranged it so I saw all my classes those 3 days. Instead of losing 3 days, I lost 2. We worked on a project those days.
This year was a transition year. We had the old test for juniors and the new test for frosh and sophomores. When the juniors were testing the frosh and sophomores were having classes as usual on a modified bell schedule. After the test my juniors and seniors watched a science documentary. Not much choice. The test often runs over, so instead of stopping at the time in the schedule it might run over 30 minutes or an hour. The class size can contract quickly. Plus the juniors, having been cooped up for the test for 3-5 hours, are in no condition to do much. So it's a free day for them. We missed 3 days for that, and a couple of days for English language arts. We reviewed for science for a week. One of those days was "my" science, and it's wasn't a bad idea. 180 school days, 10 missed for standardized tests. Could be worse. Far worse.
A poster above said there were 45 test days. These are windows. The non-English tests for the frosh and sophomores were to fit into 10 of those days. They took 3. There was very little review time for those 3 days. Most teachers took a day or two to review, but this was high school. Spanish and other non-core classes kept on going while history and math and science reviewed for a day.
On the other hand, people forget why the tests started and how they've morphed. They started with the "Johnny Can't Read" 'revelations' from the late '70s and early '80s about kids who graduated high school illiterate and unable to do arithmetic. The standardized tests were to show mastery of the basic skills. It was later nonsense that made them more and more rigorous, because obviously the teachers weren't doing their job.