Under the Welsh Intermediate Education Act of 1889, it was intended the Ruthin Grammar School should become Ruthin County School, but the Grammar School's objection was upheld by the House of Lords in 1894.
Ruthin County School at Brynhyfryd was opened in 1899 for girls only from Ruthin and Denbigh.
In 1938 the school became County School for boys and girls from Ruthin. In 1945 the school became known as Brynhyfryd Grammar School.
The school became Ysgol Brynhyfryd Comprehensive from 1967.
The collection includes: minutes, accounts, school diaries and plans.