The American Community Survey estimates that 25.75% of Americans are currently attending school according to their most recently published data in 2014. The graphic below visualizes this and shows how this differs by age group.
Mobile Users: If you are having issues using the interactive version of this visualization, you can find a static version of it here.
The data for this visualization comes from the American Community Survey which is completed by the US Census Bureau. I used the 2014 One Year Estimates Public Use Microdata Sample which can be found on the ACS Website. This visualization uses Tableau to show what percentage of the population is attending some form of school.