Evidence of an inappropriate relationship between Forrest and the girl first surfaced during a school trip to the United States in February 2012.
Two pupils approached the head of the upper school and reported rumours that the girl had a "crush" on Forrest since the trip.
Forrest denied any inappropriate relationship to senior staff and no other agency was involved, the report said.
The report said: "It is striking that it was, overwhelmingly, young people who raised concerns about this situation."
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