Some examples:

Schools with more special needs (In the literal sense) students will perform worse on standardized tests. They also need bigger budgets, because they need more staff, to support those special needs students.

Schools with more students coming from troubled homes will perform worse on standardized tests (Because not only do those students score badly, many of them disrupt their classes, causing problems for other students.) The common solutions are smaller class sizes, more administrative staff, more money spent on dealing with damage to school property, more money spent on on-site security, etc.

Being a teacher to 30 kids is hard. Being a teacher to 30 kids, and a parent to 10, and a cop to 5 is maddening.

These school districts that are spending more money, and performing poorly... Usually aren't pissing it away on serving caviar on Thursdays, and filet mignon on Fridays in the cafeteria, or on sending the Grade 5 class to Disneyland, or on buying the principal a Porche.

In general, school districts in poorer neighborhoods need to spend more money on things like subsidizing lunches and before/after-school child care. Bad school districts also can have a spiral effect where parents with means take their children out of public schools and put them in other schools, leaving the public school with a higher baseline of children with learning disabilities or other adverse conditions for learning.

School lunch programs are largely federal, aren't they? The school itself isn't paying most (or any) of the bill, and I'm not aware of any link between socioeconomic status and learning disabilities.[0] Even if it did, does having a larger proportion of learning-disabled pupils negatively effect a school's funding?

[0] http://journals.sagepub.com/doi/abs/10.1177/0022219476009009...