The schools hardly "work fine". Some do, some don't, just like some parents give a shit, and some don't. blanket statements are bad mmkay.
Ok some schools really are bad, but most are not bad. Not including obvious factors like accidental pregnancy or mental retardation, the graduation rate is most strongly influenced by parents. I know quite a few people who never graduated high school or never finished university, and the recurring theme is how nobody stressed the importance of education.
Even money doesn't seem like it has any effect on education levels. One of my uncles is fairly well off and he runs a plastic company, but 2 of his 4 kids never finished high school. One kid went to college and dropped out. Only one kid actually finished university. They lived in a nice neighborhood, nice house, they have computers, internet, access to resources like the library, good school, etc. Even with all those advantages, they still sucked at school.
I don't think it would be a genetic cause since I'm related to these people but both my brother and myself finished university. A different uncle has 2 kids; one has a master's degree and one is a dentist. Another uncle has 4 kids; only 1 of them wanted to go to university.
The pattern I see is that families tend to be all or nothing. Either most of the kids are educated or most of the kids are not educated. You can speculate what would cause that, but my guess is the parenting style.