Claudio Sanchez

We're all familiar with the term "hidden in plain sight." Well, there may be no better way to describe the nation's 6,900 charter schools.

These publicly-funded, privately-run schools have been around since the first one opened in St. Paul, Minn., in 1992. Today, they enroll about 3.1 million students in 43 states, so you'd think Americans should know quite a bit about them by now. But you'd be wrong.

Federal law does not prohibit undocumented students from enrolling in college, but it does something nearly as effective, banning them from receiving government aid. In recent years, though, some undocumented students have stumbled upon a little-known, nonprofit online university that doesn't charge tuition and doesn't care about students' legal status.

President Obama is in Tennessee previewing some of the big issues he'll talk about in his State of the Union address later this month. Friday, he'll speak in Knoxville, focusing on education and an idea that is gathering steam in some states: making community college tuition-free.

In the emerging debate over this idea, there are skeptics and there are true believers.

Rapper Pitbull (Armando Christian Pérez) is the latest in a long list of celebrities lending their star power to the flourishing charter school movement. Alicia Keyes, Denzel Washington, Shakira, Oprah — all support or sponsor charter schools.