Complex sentence: Over the course of the system's existence, about 30% of native children, or roughly 150,000, were placed in residential schools nationally; at least 6,000 of these students died while in attendance.
Split into simpler sentences: Over the course of the system's existence, approximately 30% of native children, or roughly 150,000, were placed in residential schools nationally. It has been estimated that at least 6,000 of these students died while in attendance.