Campbell Brown, the founder's son, established Tollington Boys School in Tetherdown, Muswell Hill in 1901.

Brown then opened Tollington High School for Girls in nearby Collingwood Avenue in 1910.

Situated in the middle-class, largely white suburb of Muswell Hill it was decided to integrate a large number of Afro-Caribbean and other ethnic minority children into the school from distant parts of the borough in an attempt to maximise education choice and social interaction – a policy based heavily on the United States' then-current system of desegregation busing.

In 1975, before this new intake had worked through the school, around one-third of the Sixth Form was either a first-generation immigrant, or had a surname of Cypriot or Asian origin.

Technology College status helped fund major developments to the school's infrastructure during Nixon's tenure, including construction of a new science block in 1999, a sixth form learning resource centre in 2002, and a county-standard sports hall, tennis courts and artificial pitch in 2004.

[10] He was succeeded by David Jones, who served as interim headteacher in 2005-6, before the appointment of a permanent successor, Aydin Önaç.

The governors argued that the increased autonomy from the LEA provided by foundation status would be beneficial to the school,[11] while critics argued that the proposal was an attack on the school's comprehensive nature and would lead to a reduction in provision for pupils with special educational needs.

[12] On 1 September 2007, Fortismere became a foundation school – despite opposition from 70 per cent of parents[13] and a petition from students demanding to be consulted on the changes.

[14] Önaç's reforms were marked by a conscious effort to attract middle-class children from affluent backgrounds in place of the racially and socially diverse communities that had previously made up the student body at Fortismere – he told the Evening Standard: "I estimate that a family with two children will save £250,000 in private school fees by sending them to us and [the school's sixth-form block] Tetherdown, and a family of four £500,000", and reflected that after he had been in post for two years implementing his reforms, "most families here are wealthy or middle-class".

[10] An early reform by Önaç in 2006 was scrapping the school's previous policy of mixed-ability classrooms, and "setting" the pupils by ability for all subjects, prompting accusations from parents that poorly-performing students were being segregated and neglected.

[16] Parents sought a judicial review of Önaç's approach to special needs provision, but in December 2009 abruptly he resigned from his post mid-academic year, soon after the legal action started.

[16] Önaç resigned to take up the headmastership of the selective St Olave's Grammar School in September 2010, although after his suspension and resignation there amidst legal action in 2017, a Bromley London Borough Council inquiry found that he had knowingly carried out a series of unlawful policies throughout his time as headteacher there, including the unlawful exclusion of underperforming students in a bid to boost the school's league table rankings:[17][18][19][20][21] The Times divulged that Freedom of Information requests revealed 72 A-Level students had been forced out during Önaç's tenure at St Olave's.

[16] Under the leadership of headteacher Helen Glass, Fortismere adopted a vertical tutoring system in September 2012.

[24] Some two years after Helen Glass stepped down as headteacher, co-headteachers Jo Davey and Zoe Judge revised the previous model, with a horizontal tutoring system implemented, along with a focus on grouping students by year groups, contrary to the previous college system.

A sixth form building is adjacent to the South Wing, situated just inside the Tetherdown entrance.

Finally other children are selected by the distance they live from the reception area in the South Wing building.

The following courses are offered Biology, Chemistry, Classical Civilisation, Computer Science, Drama & Theatre Studies, Economics, English, Film Studies, Fine Art, Geography, History, Mandarin Pre-U, Maths, Media Studies, French, Spanish, German, Music, Music Technology, philosophy, Photography, Physics, Politics, Product Design, Psychology, Sociology.

Students who fail to obtain a good AS-level pass, are advised to retake it before continuing the A level course in year 13.