That is a reckless statement.

It's not.

https://en.wikipedia.org/wiki/Skeletonization

"After skeletonization, if scavenging animals do not destroy or remove the bones, acids in many fertile soils take about 20 years to completely dissolve the skeleton of mid- to large-size mammals, such as humans, leaving no trace of the organism."

With a modern coffin which protects the dead body the time can be prolonged to about 80 years. But since these kids at the residential school were only buried in cheap wooden boxes ...

If they died more than 50 or 60 years ago, there will be nothing left of them anymore.

Naia's bones are intact after an estimated 12,000 years.

Because Naia died in a cave. In dry conditions skeletons can fossilize.

"Alternately, especially in very fine, dry, salty, anoxic, or mildly alkaline soils, bones may undergo fossilization, converting into minerals that may persist indefinitely."

"My" theory/hypothesis has ...

Your theory is already impossible by the simple fact that there weren't even 200 kids at the school at the time. Only 60 to 70.

No obvious specific cause.

I stated multiple causes and they are all verifiable by people who went to such schools.

Autopsy of any remains in those graves would get us closer to the truth.

The chance there are still remains of these kids is pretty much zero.

That is a reckless statement.

It's not.

https://en.wikipedia.org/wiki/Skeletonization

"After skeletonization, if scavenging animals do not destroy or remove the bones, acids in many fertile soils take about 20 years to completely dissolve the skeleton of mid- to large-size mammals, such as humans, leaving no trace of the organism."

With a modern coffin which protects the dead body the time can be prolonged to about 80 years. But since these kids at the residential school were only buried in cheap wooden boxes ...

If they died more than 50 or 60 years ago, they will be nothing left of them anymore.

Naia's bones are intact after an estimated 12,000 years.

Because Naia died in a cave. In dry conditions skeletons can fossilize.

"Alternately, especially in very fine, dry, salty, anoxic, or mildly alkaline soils, bones may undergo fossilization, converting into minerals that may persist indefinitely."

"My" theory/hypothesis has ...

Your theory is already impossible by the simple fact that there weren't even 200 kids at the school at the time. Only 60 to 70.

No obvious specific cause.

I stated multiple causes and they are all verifiable by people who went to such schools.

Autopsy of any remains in those graves would get us closer to the truth.

The chance there are still remains of these kids is pretty much zero.