This letter by Stephen Krashen, professor emeritus of education and linguistics at the University of Southern California, will be posted today on the New York Times website. I just received it:

The common core movement seems to be common sense: Our schools should have similar standards, what students should know at each grade. The movement, however, is based on the false assumption that our schools are broken, that ineffective teaching is the problem and that rigorous standards and tests are necessary to improve things.
The mediocre performance of American students on international tests seems to show that our schools are doing poorly. But students from middle-class homes who attend well-funded schools rank among the best in world on these tests, which means that teaching is not the problem. The problem is poverty. Our overall scores are unspectacular because so many American children live in poverty (23 percent, ranking us 34th out of 35 “economically advanced countries”).
Poverty means inadequate nutrition and health care, and little access to books, all associated with lower school achievement. Addressing those needs will increase achievement and better the lives of millions of children.
How can we pay for this? Reduce testing. The common core demands an astonishing increase in testing, far more than needed and far more than the already excessive amount required by No Child Left Behind.
No Child Left Behind requires tests in math and reading at the end of the school year in grades 3 to 8 and once in high school. The common core will test more subjects and more grade levels, and adds tests given during the year. There may also be pretests in the fall.
The cost will be enormous. New York City plans to spend over  half a billion dollars on technology in schools, primarily so that students can take the electronically delivered national tests.
Research shows that increasing testing does not increase achievement. A better investment is protecting children from the effects of poverty, in feeding the animal, not just weighing it.