A key aspect of the Gates Foundation's U.S. efforts involves an overhaul of the country's education policies at both the K-12 and college levels, including support for teacher evaluations and charter schools and opposition to seniority-based layoffs and other aspects of the education system that are typically backed by teachers' unions. It spent $373 million on education in 2009. It has also donated to the two largest national teachers' unions. The foundation was the biggest early backer of the Common Core State Standards Initiative.
What group has donated to the two largest national charter schools?