Four teachers who were to give long service to the School arrived in the early years of the decade: H J Bonfield (1920-58); R C Matthews (1920-52); C Ward (1920-1957); and R F Hammond (1921-51). The number of boys on the roll climbed through the 1920s, reaching 275 in 1925 and 300 in 1926. Early in the decade, a small sixth form was developed with separate Arts and Maths / Science sides. Numbers in the sixth form rose through the years until 1939, and there was a consequent increase in the number of boys going on to universities and in the development of leadership qualities in games, the cadet corps, the orchestra, drama and the arts and crafts guilds. A high standard of admission was maintained: the Headmaster remarked that it ensured that the average quality of the fee-paying boys was equal to that of the free places.

1921 saw the introduction of half-days to provide games opportunities for all boys and also the inception of the Dramatic and Debating Societies and the first masters v parents football match; at the end of the year there were 330 boys on the roll. The Natural History Society was established in 1922 and in 1923 there was a school trip to France (the first of many) and a Handicraft Exhibition. Notable events in 1924 included a PT display on Speech Day, the acquisition of a magic lantern for slide shows and 90 boys in the Cadet Force; annual summer camps for the latter had become well established.

Highlights in 1925 were a school concert and a school play; plans for a school hall and science laboratories were drawn up and the battle for funding for these additions joined. 1926 saw a continuing revival in Cadet Force numbers (membership had initially dipped in the immediate post-war period), a trip to Northern France, the consolidation of the annual Swimming Gala and the retirement of Mr Lambert, who had been on the staff since the foundation of the school in 1899.

In 1927 the quality of education provided was recognised when the Headmaster persuaded the County Borough’s Board of Education to change the name, after some debate, from Municipal Secondary School to Grammar School. When the change was implemented in 1928, the School became one of the first town grammar schools in the country; schools in Brighton, Hastings, Horsham and Rye had already made the same change  The final issue of the EMSSB Magazine appeared in July 1928 (43 issues in all) and the first EGS Magazine appeared in December of that year; the magazines started to mention what school leavers were moving on to. 1928 also saw a new cap badge featuring the Borough’s coat of arms.  The 1920s ended with construction of the new hall and science block beginning in 1929; the budgeted cost for the new wing – including furnishings – was £10,000.