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."
More top news
Thursday afternoon's weather for the east of the region
Thursday afternoon's weather for the west of the region
Thursday's forecast for the east of the region