I know this is a joke but...

The school system can, and definitely has been oppressive. They have imposed cultural norms of the dominant class or colonial rulers, and worse, have actively suppressed, punished and taught to be ashamed of local cultures.

I'm no expert and I believe it has been worst with natives in the Americas and in Africa. However, I'll consider the case with which I'm most familiar as an example.

I'm from Belgium, which has been invaded or ruled by a succession of foreign powers throughout its history (namely Gauls, Romans, Franks, Burgonds, Austrians, Spaniards, French, Dutch). Which has made the region ethnically diverse. To simplify, there are two main language families: those germanic, descendant from frankish (flemish, brabantic, limburgish...) and those romance, descendant from latin (waloon, picard, lorrain...).

At the end of the 19th century, nationalism arose and with it mandatory schooling. It was thought by the bourgeoisie that the local languages were inferior to french and that it was important that the people spoke the same language (more efficient army and economy). Hence, the pupils were forbidden to speak their mother tongues, punished, humiliated and encouraged to denounce peers. Such tactics were used in the whole of (currently) french-speaking europe (cf. the occitan "vergonha").

This idiocide didn't work with the germanic people who opposed it politically with the "flemish mouvement". The flemish bourgeoisie finally adopted standard dutch, very close to brabantic dialects, and imposed it in their region. Other languages are still in use but dwindeling due to lack of state recognition.

But it totally worked on the romance, southern region. French, a foreign language controlled from abroad is now the mother tongue of 100% of the population. Very few speak the native languages. Indoctrination worked like a charm as they now have a terrible reputation. Most people don't care about their death, and the rest is positively happy about it.