It is not a matter of spending.
The New York City public school system has one of the highest per-student spending in the world, yet two-thirds of the students can’t demonstrate basic literary and numeracy:
http://www.nydailynews.com/new-york/education/2-in-3-city-students-not-meeting-math-state-standards-article-1.2148748
It is not a matter of teacher’s pay either.
If you present-value the future guaranteed pay and benefits of New York City public school teachers, each one of them counts as a millionaire, and many are multi-millionaires. In other words, a person working in the private sector will have to be a millionaire, or a multi-millionaire, to generate from his nest egg the kind of retirement benefits a New York City public school teacher is contractually entitled to receive. (This is actually true of other unionized public-sector employees in New York City also, but only teachers are further protected by tenure, which they can get after just a few years. Oh, and they managed to get their benefits to be exempt from state and local income taxes - not a small amount considering New York’s state and city income tax rates.)
The problem is very complicated and requires that we examine our basic values, but the usual simplistic answers about spending and pay fly in the face of facts.