When it threatens the rich people with the possibility of equality or even a small reduction of their riches and superiority.

 

Much harder question. Education basically changes a wild free human into one willing to join the workforce and work for rich people for (possibly) the chance to become rich themselves or it moves them more towards just plain slavery depending on the system. Who would willing go from roaming the wilds finding food on plants and shooting wild animals with lots of free time with the family to standing on a conveyor belt building the same thing all day or in more modern worlds, sitting in front of a computer all day doing the same thing for years?