The formation of The Forest High School was the result of a campaign by a representative body known as The Forest High School Promotion Committee formed on 1 November 1958. The committee members were anxious to have established a co-educational high school that would cater for the educational needs of the suburbs of Frenchs Forest, Forestville, Beacon Hill and Terrey Hills and Narraweena.
On 31 January 1961, Stage One of construction was completed and The Forest High School opened with an initial enrolment of 318 students and a staff of 14 teachers under the principalship of Edward George Bell, who had been headmaster of Fairfield Boys' High School. The latin school motto Spectemur Agendo - let us be judged by our actions - was chosen by the pioneer students and staff, while the school crest was designed by the art teacher. Originally called "French's Forest High School", on 2 August 1961 the Minister for Education, Ernest Wetherell announced the name had changed to The Forest High School.
By 1963, Stage Three of the building program was completed with the three major buildings and enrolments had increased to 963 students. 1963 also saw the completion of the Assembly Hall in time for the second annual Speech Night on 9 April. The School magazine, "Yarrabee", was first issued in the same year - continuing to this day.
By 1964 it was considered that the school had reached the level necessary for an official opening. This occurred on Saturday 24 October 1964, attended by the Director-General of Education, Harold Wyndham, the state Member for Wakehurst, Dick Healey, and the Minister for Education, Ernest Wetherell, and was officially opened by the Governor of New South Wales, Lieutenant-General Sir Eric Woodward.
In 1969 D block and the gymnasium were opened to accommodate the growing number of students. The final stage of building was completed in 1976 with the opening of the Library.