Some disillusioned alumni of Teach for America have joined teachers’ unions and other critics who question the effectiveness of the rapidly growing nonprofit that deploys college graduates to classrooms in low-income communities nationwide, according to Reuters.

The organization founded by Wendy Kopp in 1989 will send 10,000 new recruits into schools this fall to begin two-year teaching stints. It reports current assets of $300-million, draws tens of millions of dollars a year in public funds, and has become a pipeline for education leaders, with many alumni holding high positions in state and local schools systems and advocacy groups.

Critics say Teach for America’s push for expansion is undermining the poor public schools it was established to serve, with more than a third of its troops now going into generally better-heeled and higher-performing charter schools and many being used by districts to replace laid-off veteran instructors. The organization has backed off some past claims of achieving impressive academic gains for students.