In public education today, "21st-century skills" are all the rage. Educators, business leaders, and elected officials are united around the idea that there are new skills students must have to be successful in today's economy.

Schools, the 21st-century skills argument goes, focus too much on teaching content at the expense of essential new skills such as communication and collaboration, critical thinking and problem solving, and concepts like media literacy and global awareness.

The American job market is changing, of course, and the nation does need more highly skilled workers than in the past. While plenty of low-skill jobs remain, most of the fastest-growing and highest-earning jobs do require more education and training. There are also obviously some discrete new skills that students need because of advances in technology. But, overall, none of these skills are unique to the 21st century.

Critical thinking and problem solving, for example, have been a component of human progress throughout history, from early tools and agricultural advancements to gunpowder, vaccinations, or exploration. And while "global awareness" has historically been as much a martial talent as an economic one, interconnectedness is not new nor is information literacy among elites. Likewise, the idea that there is a hierarchy of knowledge from facts to complex analysis is not a new one. Plato, for example, wrote about four distinct levels of intellect. Perhaps these were considered "3rd-century B.C. skills"?

What's new today is the degree to which economic competitiveness and educational equity mean these skills can no longer be the province of the few. This distinction is not a mere debating point.  It has important implications for how schools approach teaching, curriculum, and content.

While students should leave school with more than just facts in their head, facts do matter, too. Content undergirds critical thinking, analysis, and broader information literacy skills. To critically analyze various documents requires engagement with content and a framework within which to place the information. It's impossible, for instance, to critically analyze the American Revolution without understanding the facts and context surrounding that event. Unfortunately, state, national, and international assessments show that despite a two-decade-long focus on standards, American schools still are not delivering a content-rich curriculum for all students.

And shared content also binds us as a nation, providing a pathway to opportunity. As University of Virginia English Prof. E. D. Hirsch has tirelessly argued for more than two decades, giving all students a common framework of knowledge is a key strategy for increasing civic equality.

Unfortunately some 21st-century skills proponents believe these skills should replace the teaching of content. They believe that because so much new knowledge is being created, students should focus on how to know instead of knowing. This view threatens to reopen a debate in American education that is not new either: content pitted against critical thinking rather than the two complementing each other.

There are also real technical and logistical challenges the movement must overcome. Outside of intensive writing assignments, measuring many of these skills in a large scale or standardized way is difficult. As my colleague Elena Silva described in a recent analysis it is possible to design assessments that test both content and skills like critical thinking or problem solving. But unless these measurements are carefully designed, students can fake knowledge on many exercises intended to measure skills, again shortchanging content. In any case, most states are ill-equipped to implement such assessments today and too many teachers are not prepared to use them or teach this way today.

Ken Kay, the president of the Partnership for 21st Century Skills and one of the most thoughtful proponents of the idea, argues that the challenge today is making schools more deliberate about teaching these skills. While he values content in teaching skills, Kay sees 21st-century skills as the basic "design specifications" for a better school system.

He is certainly right that schools have been haphazard about imparting these skills to students. To the extent most students pick them up it is random, the result of lucky encounters with great teachers, schools, or other influences rather than an intentional curriculum. That must change. But it must change in tandem with a focus on augmenting—not supplanting—content and with a keen eye toward all the constraints that exist in the education system today.

The 21st-century skills movement will be invaluable if it leads to strategies to make our system of schooling more equitable  and effective by giving students, especially economically disadvantaged students, both content and various advanced skills. History, though, is not on the movement's side. In American education, the absence of any canon coupled with a tendency to run after every shiny new idea often leads to faddishness that slights the most disadvantaged students.

If they want to genuinely transform teaching and learning, proponents of 21st-century skills must be as deliberate about how their idea is approached and implemented as they want schools to be about teaching these skills.