The No Child Left Behind Act (NCLB) is an important law for at least two reasons. One reason has to do with education while the other has to do with federalism.
First, NCLB is important because it has changed the landscape of American public education. Specifically, it has put much more emphasis on standardized testing than there ever was before. NCLB demanded that all schools demonstrate that all demographic segments of their student bodies are making progress towards the eventual goal of having everyone performing at grade level. This meant that there had to be ways of determining how students were doing each year. Standardized testing would be the only way to do this that would be economically feasible and at least someone valid. Since NCLB, there has been a boom in standardized testing and a corresponding increase in frustration among many because of the amount of time being devoted to test preparation.
Second, NCLB is important because it has increased the amount of power that the federal government has over local schools. In our federal system, education is supposed to be left to the states. However, the federal government gives money to the states for education. It can withhold that money if the states do not do what it wants. Thus, the federal government exerts control over the local schools without have official authority to do so. This is something that is particularly worrisome for conservatives, who believe that the federal government should have less power. Their worries about NCLB and federal overreach can also be seen today in the opposition to Common Core.
These are the two main reasons why NCLB is an important law.