In this country we don't have a funding problem when it comes to education - some of our most funded areas are our poorest performing, and the state doing the best in improving literacy is
Mississippi, which is not exactly our richest state (nor did they dramatically increase their spending - they instead managed to adopt reforms that have been
opposed by Teachers Unions).
There is no Single Cause of our education problems in America. Complex outputs have complex systems that lend themselves poorly to monocausal explanations. Repairing our family formation would help. Having elites preach what they practice would help. Changing cultural narratives about the locus of control in our lives would help. School Choice would help. Phonics (and other innovations) would help. Ending the extent to which the system is run by one particular class of people (middle class women with masters' degrees) would help, as would reforming our system to focus less on preparing kids for college (those who go to college generally need support the least) and more on preparing kids for a broader range of life choices (or, better, allowing diversity of education experience so students and parents have greater ability to choose). Shifting our spending from administration to teacher pay (allowing us to attract higher quality candidates) would help, as would public sector union reform.
But we can generally (at the system level) rule out one problem - total expenditures. We're good on that.