Of course, maybe they had a point, because they didn't want Nog exposed to the propaganda of Marxism-through-high-technology.
I'm pretty sure the Ferengi have an extensive and effective education system in place - one based on learning at the job. And one of its main features is going to be inequality, the emphasis on some Ferengi getting the drop on others by cunningly acquiring better education. Common schooling would be the ruination of that, never mind the embedded UFP propaganda that Nog would be contaminated with if he were allowed to be schooled by Keiko O'Brien.
It's not as if anybody said Nog didn't need the skills of reading and writing, or that he wasn't going to acquire those in the course of a typical Ferengi childhood. Certainly the Grand Nagus didn't need to be explained what a "school" was; nor did he object to the practice of schooling.
Really, the only one who says Ferengi don't need schools is the moody Nog himself. And he's probably not even really serious, merely mocking his father's comments.
I also suspect learning a xenolanguage would be very difficult, even as a child. Forgetting the obvious things like lack of cognates and a totally alien script, it wouldn't even be a natural language, at least as far as the learner is concerned.
But all that would be true of the child's native language, too. Even the dullest Chinese child can learn Chinese (at least one sort of it, and probably several)... And would continue to be true through his or her life, really. It would basically just be a matter of having the right sort of environment around the learner (say, native speakers of the language), and doing the learning at young enough an age.
Who knows where the age limit goes on a Ferengi? Or on a human, for that matter; 24th century science might have unlocked the secrets of lifelong learning, potentially a much lesser feat than those nifty doodads of Crusher's that remove headaches...
Timo Saloniemi