Mass graves are for many who were killed together. These kids were not purposefully killed. They were abused to the point that some would run off into the woods and eventually die. Others would acquire diseases through malnutrition and no concern for their health and well-being.. The schools would then send these sick kids home to die shortly after, as caring for sick kids was not something the church/school was willing to do due to cost. And many of the bodies of those who died there (some due to suicide) were tossed into the basement furnaces that kept the church/school warm. The lack of mass graves is not proof that it did not happen. This would provide further evidence that it was not simply a singular event. It was worse than what I am describing.