The no child left behind act was enacted in 2001. If you check the article, it has a nice little chart, showing a decline that starts in 2015. Prior to 2013, the results show a clear trend of improvement (in regards to the percentage of students achieving a minimum level of proficiency).

How would you explain that temporal gap? If the No Child Left Behind Act is the problem, why was the trend positive for the first 12-14 years of the time it's been in force?

Gifted programs dropped from ~72% of elementary schools to ~65% by 2013, and probably have continued declining. Given it takes 10+ years to educate a child, the school culture to change, and so on, we should expect to see quite a lag between policy and outcomes.