I have never been to a US K-12 school, but I was a lab instructor and teaching assistant for about 200 undergraduate students when I was a graduate student at University of Minnesota. I was very surprised that 20 year old engineering students were unable to do simple calculations in their mind. I asked them what was the reason. They told that they were allowed calculators from their primary school. They were asked to do the calculations on their own and then "check" their answers on calculators. I can't imagine most people "checking" their answers on their calculators.
So, the one thing that I would change is to remove calculators in K-12 education. Then, the math standard in US schools would improve.