Common Core has contributed greatly to decline in academic achievement. K-6 teachers use it as a guide, and feel that it is more of a ceiling to reach rather than a basement to stay above. I found that as I continued teaching high school math, I had to gradually make my tests a bit shorter as students seemed not to be able to finish as easily as they did when I first started teaching. I also noticed that students were coming to high school unprepared to do basic arithmetic, relying heavily on calculators. Number sense just wasn’t as strong as it needed to be — e.g. multiples of 12 or 15 were not readily recognized. Junior high math teachers — often K-8 generalists — would argue that students don’t need to simplify radicals — just give ’em a calculator and round to the nearest hundredth (eyeroll…). For example, something like the square root of 12 over 4 simplifies to the square root of 3 over 2, a frequently used trigonometric ratio. If you haven’t taught trigonometry, you probably won’t recognize how important that kind of number could be.
It always felt like the teachers at the K-8 level felt that they should be the teachers who determined what needed to be learned in math and to what level of expertise. I once gave a presentation to a group of K-8 teachers and gave an example of the basic fact 9 x 7 = 63 and how it was used in all high school math classes through AP calculus. I’m sure that many didn’t understand the examples I gave, and it not, I rest my case.