My daughter passed for Loreto grammar and I think the prep is a lovely learning environment.
Your statement was not quite true as I remember several girls joined the school late, so did not have 7 years at the prep. Possibly a problem existed before they arrived! Most of these girls have been successful in getting into good independent schools. Not all parents want their children in the state sector. I think your figures have been somewhat exagerated too.
Getting 100% success rate into the grammar is not difficult. Just ask the weaker pupils to move schools beforehand which is what many prep schools do. I know of several friends where this has happened.