Any debate about fairness in school funding has to start with clear data, but it turns out that data can be very hard to find. Most people understand the inequity of school districts in wealthier areas having more money than those in less affluent communities. Addressing these disparities — across school districts — is important; but just as important, and less understood, is the unfair distribution of resources within school districts.

Part of the difficulty in tackling these in-district disparities is that — outside of a few spare calculations from nonprofits and the federal Education Department — parents, local advocates and even school board members are often in the dark about how spending breaks down across their local schools.

The Center for American Progress conducted a rare study of this issue and found that at least 4.5 million students from low-income backgrounds are in schools that receive roughly $1,200 less per child each year than wealthier schools in the very same district.

In a single school, this shortfall can be the difference between adding 12 counselors, granting thousands of dollars in bonuses for dozens of teachers who transfer into hard-to-staff subjects and offering high quality art, music and extracurricular programming — or adding nothing at all. And these baseline funding numbers do not even include major advantages affluent schools have through well-funded P.T.A.s, parental social capital and connections with colleges and universities.