Some anecdotal (but true) evidence:
This information is from the experiences of a Sioux family who grew up in South Dakota in the 1940's, a family I know well. The names have been changed to protect the innocent.

Juanita and Ophelia were tortured for speaking Lakota instead of English. The punishment for this "crime" was to be beaten with shovels. For a different infraction, the girls were forced to kneel on the same shovels. As mentioned by sid, forcible hair cutting was not considered punishment, but it was a terrible desecration of identity.

Juan spoke of the sexual abuse they endured. In his own words, "When night came, all you could do was lie still and pray it wasn't your turn." The incidents of abuse were reported, not years later by adults, but by the children.1

When Juan died, the nuns who used to beat his sisters came to the funeral. They made rosaries and prayed for his soul.

In an interview on The Early Show on 6/12/02, a vetran of WWII, one of the original code talkers. stated, In the government schools they would wash our mouths with a brush and brown government soap for speaking our language. Ironic. Twenty years later, the government needed our language to win the war.This is a paraphrase but is as accurate as I could get it.
1: "Nobody paid any attention until white people started to speak out." -- Juanita