This is not unique. In fact it seems to be common in many catholic institutions around the world.
In Ireland there have been several (some ongoing) Government investigations into these places. Timeframe is generally c 1860s to 1960s.
Generally there are two types of institutions with mass graves:
1. Industrial schools - orphans or kids who's parents couldnt care for them.
2. Mother and baby homes - places where teenagers who got pregnant went to stay until their child was born - child was usually left in institution (often against the girls wishes) and trafficked to be "adopted' by rich catholic families in the US - usually for a large fee to the nuns who ran these homes.
Conditions in both types were terrible, with disease and mistreatment - very high mortity rate. The religious usually didnt care about the dead bodies of deceased - in one case in Ireland they were flushed into a septic tank. There normally would be death certs, but often with incorrect cause of death recorded by institute doctors to cover abuse.
