As was mentioned in the previous post, Napoleon is teaching the younger pigs in Chapter Nine that they are superior to other animals in order to ensure that pigs remain in the upper class on the farm. Napoleon indoctrinates the pigs privately in the farmhouse kitchen, and discourages them from playing with the other animals. He even creates a rule where other animals must stand aside and make a path whenever a pig walks past them. By privately teaching the pigs at a young age that they are better than other animals on the farm, Napoleon encourages prejudice, which gives future pigs an advantage. Exclusively educating the pigs also creates an unbalanced, unfair society where all pigs will be given leadership roles as well as opportunities to make laws in their favor. Young pigs will grow up believing that they are better than other animals, and will be given leadership roles simply because of their "race."
The young pigs are the only animals being educated in the school house because it is Napoleon's way of elevating the race of pigs on the farm. If only pigs are educated then he can ensure that only pigs will have an elevated place in society. If the other animals were allowed to educate their young then it runs the risk of a different animal being smart enough to take a leadership role in the government and management of the farm. Napoleon does not allow the young pigs to play with the other animals because he is teaching them that they are different, essentially that they are too good to play with other young animals and that they need only to associate with their own kind. In essence he is teaching the pigs to be racist toward the other animals. The young pigs will grow up their entire lives with other animals stepping aside for them on the pathways, wearing green ribbons on Sundays, eating better food, and they will be the only educated animals. They will have no choice but to view themselves as better than all the others.