As a kid who wasn't so poor, but did have a few poor friends, I can point out some anecdotal differences: My parents had time to show interest in my school activities, discuss what we were learning, and find out if I needed help. When I couldn't understand centripetal force to my satisfaction for my 5th grade science fair project about the yoyo, they invited their NASA friend over for dinner so he could talk about it with me. I never wondered if my parents could afford to buy school items. I never worried about my parents being in a bad mood because they lost their jobs or couldn't afford to put food on the table and go to a relative's funeral. (I remember being at a friend's house while his parents were fighting about this.) I had a friend whose parents were very poor when her father lost his job, he was in a horrible mood for weeks and then turned to alcohol. Growing up in that kind of environment is incredibly stressful. I didn't understand it so much as a little kid, just later. For me, getting a B was a horribly stressful event. My parents might be disappointed and I might be stupid. For my friend being home was a horribly stressful event. Grades never mattered at all to her. Her parents didn't have time to pay attention to her schoolwork, they had other more important things to worry about.

My wife was briefly a teacher on an Indian reservation. There were few jobs on the reservation and many of the kids' parents were unemployed, and even the ones that were rarely were in a position that required any education. That's a huge demotivator for kids--why sacrifice for education when you can't see in front of your face that education might yield any benefits? Then there was, of course, the social ills associated with poverty, which distracted kids from school: alcoholism, drug abuse, crime, domestic violence, and sexual abuse.

I'm personally enormously skeptical of the idea that education is a solution to poverty,[1] at a large scale. There is a game theory problem in play. The fact is that it's unlikely that education will lift an individual inner city kid "out of the hood." A relatively excellent outcome for diligence and hard work would be going into debt to attend a third rate college, for the privilege of fighting for a low-paying service job. So it's totally rational for kids to be more preoccupied with whether joining the right gang will keep them from getting harassed on the way home from school.

But if everyone worked hard and got educated, what might happen is that economic opportunities could be created "in the hood." That's the prisoners dilemma--the individually rational decision to devalue diligence and education leads to a globally worse outcome. This is where culture comes in. You see this with poor immigrant communities. They have little capital, but have cultural mores that create an incentive for education and hard work. A kid might not leave the neighborhood through education, but he'll get social standing in the community, among authority figures and peers. When everyone has that incentive, that creates economic opportunities within the neighborhood. After all, there are neighborhoods in Bangladesh far poorer on an objective scale than the worst ghettos of Chicago, that nonetheless have bustling local economies.

[1] Poverty is, of course, relative. But I'm not talking about utopia, but just about raising the plight of the poor here in the U.S. up to that of some places in Western Europe.

I grew up relatively poor (Appalachia 1980s/90s).

What made the difference for me were two things.

1) Parents that were role models. My mother didn't earn much money, but she had a tremendous work ethic (she grew up on a farm), and was a very disciplined person in terms of morality. She was always present in my life, and was quite firm and consistent in terms of discipline (if you did something wrong, you were punished; and you listened or else). My father was an entrepreneur, very creative, very intelligent, high ingenuity, exposed me to a lot of amazing experiences growing up (traveling, ideas, technology, etc). Neither parent drank, had drug problems, or had ever been arrested. I had a consistent, safe home environment, where being smart was encouraged, independence and success were outcomes to strive for.

2) A tremendous public education system, with high quality teachers across the board. Almost all of which were older women interestingly, most were very firm, attentive, and persistent. The school itself might have had several hundred students, but from K through 5, I had one male teacher out of maybe 15 or 20, and only one younger female teacher.

The kids that did the worst in school, almost universally had very bad home lives. They were not only extra poor, but they had terrible parents (or absentee parents); abusive, alcoholic, non-working, etc types.

Maybe because parents and home environment matter as much, if not more, then teachers?

Middle-class and upper-class parents who work 9-5 jobs have the time to spend playing and teaching their kids on nights and weekends.

They have the money to buy good, nutrious food.

They have the knowledge (either from their own, self motivated research, or from their friends who do the research for them) to do things like limit screen time and not use a TV as a baby sitter.

Poor people can do all this too (and many try and do), but being poor makes it harder, which is one of the reasons economic class is inherited.

All I can do is offer hypotheses.

I suspect it's death by a thousand cuts. Some broad categories: 1) What happens to kids before they ever reach school age, and the environment they experience when they are not in school. 2) Though teacher quality is only part of the equation, teachers compete to transfer out of the poorest districts. 3) Lower expectations from school leadership, parents, and community -- possibly inadvertent due to not even knowing what high expectations look like.

My kids are in a public school system that's believed to be of very high quality. Parents bid blindly on houses to get their kids into this district. We probably have one of the highest percentages of parents with advanced degrees in the country. The "lucky" kids start kindergarten knowing how to read, are coached in math at home, may be learning more than one language, taking music lessons, eating right, playing outside, etc. Parents have time to volunteer at school, which is not just an added supply of labor, but also helps reinforce the kind of learning atmosphere that educated parents value.

I don't think there's a simple answer to that. There are a mind-boggling number of factors at play and no major study, to my knowledge, has isolated poverty's covariants in a way that would let us say which specific factors outside of poverty itself are the primary drivers of poor outcomes among the impoverished.

Because general education, not apprenticeship, is for someone who has their minimum met already. Education which is supposed to benefit you in 20 years is hard to appreciate when you are worried about tomorrow's meal, rent.

Now of course someone will say "kids, whether rich or poor, do not think about 20 years down the line." They would be correct; however here the success of a kid's education depends significantly on their parents' contributions to it. If a parent is poor, and worried about their next meal, you cannot blame them for not having the energy to worry about the long term.

The things that keep parents poor also prevent them from helping educate their children. I spend more time looking at those that overcame poverty. We often see cases where it was parental pressure on the children to do well in school, and placed a value on educational achievement. In many cases, the ego of a parent who did not do well in school leads them to devalue educational achievement in order to feel better about themselves...

The short answer is "We don't know for sure"

The long answer is "We don't know for sure, but it's likely that all of the replies to your question have some effect"

Probably because most schools are public schools which are funded by local taxes.