The Foundation has its origins in the parish of St Dunstan-in-the-East,established on St Dunstan's Hill. A christian church has probably stood on this site since the Roman occupation. The oldest remaining written record dates from 1272, the vestry "Grete Book", having been lost in the Fire of London.

In 1446 a writ of Privy Seal addressed to the Chancellor, stated that the Archbishop of Canterbury and the Bishop of London, "considering the great abusions that have been of long time within our citee of London that many and diverse person not sufficiently instruct in gramer presumynge to hold comune grammer scholes, in great deceipte as well unto theire scholers as unto the frendes that fynde them to schole", had "of their great wisdome set and ordeigned five scholes of gramer and no moo, one within the church yard of St Pauls, a second at St Martin Le Grande, another at Bow Church in Cheapside, another at St Dunstan-in-the-East and the fifth at the Hospital of St Anthony." The Chancellor was ordered to "Command all lige subjects not to trouble or impeche the maisters of the said scols, but rather helpe and assiste inasmoch as in them is."

 By 1865 it was agreed to use the majority of the endowments of the parish to provide a new school on parish land elsewhere. "Owing to the enormous prices of London ground suitable and the very signifcant tendancy of large London schools to move out from the centre, the school should be placed at such a convenient distance from London as to be accessible by railway and that it should therefore be built on the Charity Estate at Catford Bridge in the Parish of Lewisham contiguous to a station on the Mid-Kent-Line of the Southeastern Railway Company, whereby proper play grounds would be secured for recreation and the remaining portion of the estate very greatly improved for building purposes...to engage first class talent in Masters and to supply First-Class education to the pupils adapted to the circumstances and the wants of the present day.