When my children were in public school they were both in the Enrichment classes and all just rolled on well, I had no idea about the Common Core standards butI knew the order in which they were learning math seemed different to how I learned, like they skipped around...a lot.
They both were A students, then when my daughter was in 6th grade she started at the military school. It was public independent, a designation which meant they did get state funds but we also paid tuition.
She started struggling in Math, not mightily, but her A's were becoming B's and I scheduled an appt. with the Math teacher. She said she had seen this decline for many since the Common Core was instituted. The military school didn't follow common core standards and a lot of the kids were struggling. The enrichment kids even more so because enrichment was all about creativity and abstract thought so deadlines and structure weren't as ingrained.
I hired a tutor for her and they started filling in the gaps of what the military Math teacher said the common core students often lacked and she caught up.
C was a more difficult case, as long as he was in the classroom he could follow the work at the military school but he couldn't retain it because his elementary math wasn't taught by laying a foundation then building blocks but by skipping around. So he may know how to carry out a math function but not how it may relate to something else because it wasn't learned in the order that it should've been.
One more thing, I remember my first college math classes, Alg. 101 or 102 then upward. I was surprised that it really isn't a core curriculum for college any more, there is mathematical decision making, mathematical modeling, statistics, pre-cal and the ALG classes are now 97-98 classes and do not count toward graduation.