A Typical Classroom
The idea behind a national curriculum is to standardize the education of all the students in the nation. In relatively small countries – i.e. with little land mass or low population – this isn't so much of a problem. Generally, the national government in these countries has control over the entire country and thus has the power to enforce whatever curricula it desires.

In larger, more segmented countries like the United States, this becomes a bit of a problem. The federal government may introduce a national curriculum that it wants every school in the country to follow, but there is always the possibility of state governments stepping in the way. What this means is that it's very difficult, not just to implement a federal curriculum, but to enforce one.

In the United Kingdom, in 1988, the Education Reform Act was passed. This implemented a national curriculum in England, Wales, and Northern Ireland. It dictates the subjects that should be taught at certain key levels, called Key Stages. In total, there are four key stages in the curriculum, with the fourth culminating in an exam called the GCSE (General Certificate of Secondary Education), which is somewhat equivalent to a high school exit exam in the United States.

The closest thing to a national curriculum the United States has been able to implement thus far is embodied in the No Child Left Behind Act of 2001 (NCLB). This act of congress instructed states to implement standardized testing for all schools in their jurisdiction. Unfortunately, this does little to actually level the playing field across the nation, as each state could have wildly differing views on what standards their education system should be held to.

NCLB has, in fact, lowered the overall quality of education in the United States because it holds the public schools accountable for low test scores in the worst possible way: by removing funding. That is, if a school's average score on the exam is under a certain benchmark, the school loses federal funding, which in many cases, leads to the closing of the school. When the school closes, of course, the students are relocated to other schools in the area, leading to overcrowding of the other schools, which lowers the effectiveness of the education just that much more.

As you can see, there are two very good examples of the dos and don'ts of national curriculum. The idea of implementing a set educational system for a country is great, but if the extent of that system is a test at the end of the year, what you end up with is a bunch of teachers that stop teaching the material and start showing the kids how to pass the test. The end result of the latter is that students come out retaining very little of the information they were actually supposed to have.