Thetford Grammar School traces its origins to AD 631 when it is likely that Sigbert, King of the East Angles, provided a school for his court in Thetford.  Less  conjecturally, a document of 1114 under the seal of Herbert Losinga, by then Bishop of Norwich, records that:

"I have restored to Bund, the Dean, his schools at Thetford as completely and advantageously as he ever held them".

It is likely that those schools were run, possibly under the aegis of Losinga himself when he was still Bishop of Thetford, within the precincts of what was, at the end of the eleventh century, the East Anglian Cathedral. This cathedral occupied what is now the site of the Old School.

"The teacher should studiously govern his pupils by example, rather than teach by manner of words."

Herbert Losinga

The school’s Roll of Headmasters, unbroken since Bund’s time, testifies to the school’s medieval history, with the Duke of Norfolk, victor at Flodden Field, among its pupils.

Sir Richard Fulmerston was responsible for ensuring the school survived the Reformation. The  refoundation was confirmed in 1610 with the ratification of Fulmerston’s will by Act of Parliament.

The school continued in its one-room Elizabethan building, the accommodation more or less unaltered for three hundred years. Its pupils included Pepys' contemporary Roger North – lawyer, historian and musician – and the radical polemicist Tom Paine.

"The world is my country; to do good is my religion".

Tom Paine

The 1880s saw major developments in the fabric and philosophy of the school under Benjamin Reed and Reed’s school was known as the best in Norfolk.

"In all this work the Assistant Masters have taken great interest and worked hard to make the school life a joyous as well as prosperous one".

Ben Reed

In 1888, it was joined by the Victorian Girls’ Grammar School, built across the road, in part with money left by Sir Joseph Williamson, Secretary of State to Charles II and a former Thetford MP.

The two schools continued to grow and thrive through the twentieth century, adopting Voluntary Controlled Status in 1944 and forming a single coeducational establishment in 1975.

The school  returned to independence in 1981, rebuilding itself as a small but academically ambitious school which at the same time pays attention to the "wider curriculum" – a contemporary orthodoxy which has clearly, however, always been part of its long tradition.  This ethos is reflected in the continued development of the school into the C21st.

The Losinga building was redeveloped in 1998 to accommodate the growing technological needs of the modern age.  But the school was reminded of its medieval roots during the construction of the Cloisters Sixth Form Centre in 2007 when extensive archaelogical work confirmed the of the last resting place of members of the Dominican Friary.