Penn Foster College is a large online school, but whether it's the *largest* independent accredited school is difficult to definitively say without a comprehensive, regularly updated database of all such institutions worldwide. There's no single, universally accepted ranking that tracks this specific metric. The definition of "independent" can also be nuanced.
While Penn Foster is a significant player in the online education space, determining the absolute largest would require a vast amount of data collection and analysis, factoring in student enrollment, accreditation type, and the precise definition of "independent."