Oftentimes it's a funding issue, but not always. Even in large metropolitan cities where each student pretty much receives the same amount of funding, there are still massive differences in the quality of education. Unfortunately the issue goes much deeper than funding and involves issues like the wealth of the surrounding neighborhood, the culture of the students and their parents in that neighborhood, etc. Even if two schools in the same city get about $10k/year for each student, if one school has a median family income of $100k and the other a median family income of $15k, there is going to be an absolutely massive difference in educational outcomes.