That is an official undercount, as it is only having checked 65 of the 400 schools in existence.

An academic estimates based on population losses and speaking to the families who have lost children, that over 40,000 perished in the system.

This is also only graves found, as many children were lost in transit or got lost fleeing the schools. Here's David Swallow Jr. (Oglala Lakota) explaining his experience with the schools and running away. He was fortunate enough to know how to get home.