A little Sioux Nation (Lakota, Dakota, Nakota) history research will show you that during that time the bands were no where near OK, TN. In fact they were nowhere east of Minnestota or south of Nebraska. Today that is a different story.
The Indian Boarding Schools (a horrible story in itself were Indian children were forcibly removed from their homes and sent 100's of miles away from families) were introduced in 1879 with the first located at Carlisle, PA. Later other's were added. Chilocco Indian School. OK was est. 1884 and would be the first introduction of Sioux in OK. Many Sioux children were sent to both places, but most returned home either by running away or at the age of 18 or they were sent to be housed in a white household (mainly young girls) elsewhere other than 'home' to continue forced assimulation.
You are not specifying years, but I assume it was long before the time frame above.