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
A 72-year-old man has been convicted of 15 counts of indecent assault on boys in a care home between 1976 and 1983.
Special bus timetables will apply over the Christmas holiday in the Reading area
House prices in the south-east are expected to rise by 5% in 2015, fuelled by the recent change in Stamp Duty tax rules