Common COre is a strange thing. Its good and bad. If you are a weak school it brings out the problems. You can see a standard that you must achieve. However if you are an above average school forget about it. The education will bring you down.
The reality is that there is no common. Not in any state, and not in any classroom. Kids are different. And they should be educated in a fashion that allows quicker kids to move at their rate. And slower kids need the help they need to get to an acceptable level. Remembering that every kid can’t get to an acceptable level. Thats why software is so helpful. Kids can move at different rates.
Another factor is the reliance on testing. Many students will struggle, but then at some point it will come together if you nourish their confidence. The constant pressure to pass a test short circuits that process.