No one is “too difficult to teach”, but statistically kids from underprivileged backgrounds have a lot more hurdles to face and a lot of catching up to do. They are much less likely to have gone to pre-school, which means they were less prepared when they started kindergarten and had a harder time learning to read, learning simple math, etc. And once you start off behind, it is very hard to catch up. The problems multiply as the kid moves up in grade level. In addition they are less likely to have a quiet, safe place to study, they suffer more stress in general, which makes learning harder, they’re more likely to be undernourished, which also makes it harder to learn, and as they get older all this only gets worse. They are more likely to have to find a way to earn money, interfering with their schoolwork. And I won’t even go into issues like not having education supported or treated as a valuable item by their parents, let alone respect for authority, since that gets controversial. So the underprivileged kids will be harder to teach. But that just means they need you more, and the ones who put in the work to get past the problems will make your job all the more rewarding.