The original school on the site of the Glenunga International High School (previously called the Glenunga High School) opened in 1963 as the Adelaide Technical High School. Adelaide Technical High School had formerly been located in the Brookman Building on the corner of North Terrace and Frome Street in the city. The school had been established as the Preparatory School of the South Australian School of Mines and Industries in 1903 with an initial enrolment of about 90 students. The objective of the school was to prepare students for School of Mines courses as well as providing suitable secondary schooling for those wanting to enter directly into commerce and industry. While initially the school catered only for boys, girls were admitted in 1915 and a commercial course was provided. The school continued to operate under the direction of the School of Mines with one or two name changes until the Council decided in 1918 that it would be known henceforth as the Adelaide Technical High School.

From its earliest days, entrance to the school was based on academic merit. Prospective students had to undergo an entrance examination with the acceptance standard based on the highest grade of English and Arithmetic taught in Primary Schools. This had the advantage of providing a means for limiting the number of students to the available accommodation in the building which also had to cater for the needs of the School of Mines.

This shortage of accommodation remained a problem for the school for the entire duration of its stay at North Terrace. With the expansion of the School of Mines due to the establishment of new Degree courses in the 1950’s, the Council decided that the school would have to transfer as a complete unit to a new site. However, no suitable new site in the city could be found and it looked as though the school would have to close. Last minute negotiations with the Government of the day then resulted in a decision to establish a new school at Glenunga. As part of the change, the school was to come under the direction of the Department of Education and all connections with the School of Mines (by then known as the Institute of Technology) would be severed.

The construction of the new school was completed in 1963 and the whole school transferred from North Terrace in September that year. Over the next few years the specialised curriculum of the old Adelaide Technical High School which had been developed to suit particular objectives gradually changed to a broad-based curriculum similar to that available at other high schools. This was reflected in the change of the name of the school to Glenunga High School in 1974. With the introduction of the International Baccalaureate course in 1990 the name of the school changed again and since 1992 has been known as the Glenunga International High School.

In March 2015, the Blue and Gold Alumni Association was launched at GIHS with the objective of providing a means for former students to keep in touch and to collect and maintain the memory of a unique school which made a significant contribution to education in South Australia. As part of this objective the Association funds two scholarships for students of the Glenunga International High School. It is hoped that the activities of the Association will inspire Glenunga old scholars to continue with and expend these objectives for the long-term benefit of the school.

Membership is open to persons who have attended or taught at Adelaide Technical High School, Glenunga High School and Glenunga International High School.