The school was founded by German-British educator Kurt Hahn on 13 October 1963 with funding received from the USAID, Ford Foundation and donation of landspace by the then Nigerian Western Regional government. Most of the pioneer teaching staff were British expatriate educators from Gordonstoun in Scotland. The school is a co-educational Boarding and Day School admitting pupils aged 10 to 16. It was primarily established for children of expatriates, living and working in Nigeria. It opened its doors to pupils of both expatriates of diverse nationalities and highly placed Nigerians. The first Principal was David S. Snell of blessed memory; followed by John Gillespie. The longest serving Principal was an Anglican clergy, Archdeacon J.A.Iluyomade of blessed memory. He was also the first indigenous head of the school. After him was Rev. Dapo Ajayi also of blessed memory, then Dapo Fajembola also of blessed memory. Thereafter came the first female Principal, Esther Adetola Smith. After her was R.O. Akintilebo, Dr. M.B Malik and Phebean O. Olowe ISI is well known for socials, extra-curricular activities like the international soiree. The International soiree is an evening where all the nationalities represented among staff and students display the food, clothing and other material aspects of culture for sale with the objective of raising money for the less privileged. The Charity walk is another event that students use to raise money for the less privileged. Service to mankind is embedded in the culture of ISI. This tradition continues till this day. The school song goes thus:
Academically, it originally followed the British system of five years to preparation for Ordinary Level and West African School Certificate Examinations; with another two years in the sixth form to prepare for Advanced level and Higher School Certificate examinations. As a result of the introduction of the 6-3-3-4 Nigerian educational system introduced in the late eighties, it runs the mandatory six years of secondary school education, in addition to preparing students for international exams like the annual IGCSE 'O' Levels and Cambridge 'A' level exams. It also for many years has offered preparation for the International Baccalaureate. Although it now follows the 6-3-3-4 system of Nigerian education, many of the students continue to prepare for international exams. The Cambridge 'A' Level programme was recently resuscitated in September 2011. The School celebrated its golden jubilee in October 2013, with a fund-raising for the building of the much needed school hall. The hall is yet to be realised, mainly due to insufficient funds.
