The school was founded by Geoffrey Glyn, Doctor of Laws, who had been brought up in Anglesey and had followed a career in law in London. A friary had been established in Bangor by the Dominican Order, or Black Friars, in the 13th century. At the Dissolution of the Monasteries, the friary was wound up in 1538. Geoffrey Glyn bought the site with a view to establishing a Grammar School. In his will dated 8 July 1557, he left the property and endowments towards establishing the school.