Eight boys made up the first group of students, and were taught English and Classics by Michael C. Howe, a scholar from Trinity College in Dublin, Ireland.

He was not an engaging instructor, and typically spent the school day seated at his desk reading a newspaper and inhaling snuff while his students taught made the effort to teach themselves.

Enrollment at the school reached between 250 and 300 boys within a decade, and roughly 80 percent of attendees were from elsewhere in Canada, the United States, England, and the West Indies.

[4] To accommodate the growing number of boys attending the school, a one-room schoolhouse was built on the present site from land donated by the Dickson family in 1853.

Additions were made to the original building as required, and by 1870 the school's layout resembled a two-storey stone cross.

Tassie stubbornly refused to change his teaching methods, based principally on rote memorization, and consequently his students consistently failed to meet provincial standards.

By 1881 the enrollment had fallen to 50 boys, and Tassie faced increased criticism that he ultimately resigned that year along with his entire staff of teachers.

[8] Sarah Crawford was the first woman to become a member of the teaching staff at the Galt Grammar School, and taught French to the boys from 1870 to 1877.

Tassie was pressured to adopt co-education and allow girls into his school to receive the same level of education as the boys, but refused.

As a compromise, he established the separate Girls' School, which officially opened on April 15, 1872, in the former Wesleyan Chapel on North Street.

When Tassie resigned in April 1881, girls were admitted attendance at GCI with the boys and the building was sold to The Crown for $750.00 and became the Galt Armoury until about 1914.

[10] Tassie was succeeded by John E. Bryant, who was tasked to hire a completely new teaching staff and revive GCI's prestige while embracing educational reform.

[12] Bryant encouraged sports, believing that they provided "the antidote for mischief present in the composition of every true boy," and organized the GCI Football Club.

In 1925, the Galt Staff Players Club was organized and performed its first play, Bayard Veiller's The Thirteenth Chair in February 1925.

A student publication known as Specula Galtonia, the forerunner of the school's present-day yearbooks, began to be produced for a time until the Great Depression.

Both programs were vital to providing the Canadian Navy and Air Force with enough skilled mechanics to properly maintain engines and machines.

Seventy-eight men from GCI ultimately lost their lives, and their names were later added to the memorial tablet in the main corridor.

Students attending GCI are eligible to be recipients of a wide variety of awards and scholarships from generous donors from alumni and the community.

Richard Guisso, while attending St. Michael's College at the University of Toronto for a major in history, was awarded a Rhodes Scholarship for 1966.