The Board of Education then withdrew its notice.So it was that Arthur Hall Prowse, the Second Master, was appointed Acting Headmaster, to guide the school through a difficult time. Since he was not a graduate, he was warned that he had no chance of the permanent position that would occur when the county's plan of merging the Monoux School with the Walthamstow County High School for Boys came to fruition. The latter school was the Technical Institute, taken over by the Essex County Council by virtue of the 1902 Education Act, and renamed about 1912. Two reforms initiated by Prowse were the house and prefectoral systems (l915/6). Both of these had been orally recommended by the 1913 inspection The houses were simply numbered at first from one to four. They were named shortly before the move to Chingford Road and expanded to six. This was reduced to four again in 1974.

The staffs were amalgamated and the two schools became one, at least in theory, on 1 September 1916. The County made additions to the High Street building at a cost of £1,400. These were a series of hutments made of corrugated iron, other materials not being available during the war. In fact, the Hoe Street boys did not attend in the Monoux buildings until 7 January 1917. In December 1916, an unprecedented number (five) of new assistant masters had been engaged and in the previous September a new Headmaster, G. A. Millward, had taken up his post. The High Street buildings therefore became even more
cramped.

The school was not entirely taken over by the County in 1916. It had an unusual status, according to the Board of Education, of "Independent School voluntarily controlled by the Authority". This lasted until 1920.

After amalgamation boys were still admitted at the age of eight, though this was phased out just after the war. Christ's Hospital entrance took place until 1923. Gradually, however, eleven plus became the usual entry age for the Monoux School.

Even before the amalgamation, in October 1915, a new school building had been decided upon. On 7 November 1916 the Essex County Council initiated an abortive Compulsory Purchase Order for the new site in Chingford Road, even before the High Street extensions were ready. The new school was to be for 300, but capable of taking 400 if necessary. The site was finally bought on 23 July 1919 and in October 1919 the Authority leased it to Mr. Hitchman for grazing cows since "no works were in prospect this growing season".

The period of amalgamation saw problems in the leadership of the school. G. A. Millward, the new Headmaster, joined the Forces in May 1917 and, until his demobilisation in January 1919, Prowse again led the School. Then Millward left in order to take up the headmastership of King George V Grammar School, Southport, and was succeeded by J. K. King.

September 1920 was a difficult time for a new headmaster to take up his appointment. The Geddes axe was in progress and the promised new school seemed as far away as ever. The country was in the first post-war depression, with high unemployment and social and industrial unrest. King has all but been forgotten as a Monoux headmaster, but it was he on the one part, and the County Council's subvention on the other, that turned the school into a first-rate institution. The Cambridge Local Examination was rejected in favourof the London Certificate; the teaching of Spanish, and regular teaching of German (sporadic from 1912) were introduced. The Cadet Corps was disbanded, proper games and physical training - the latter in hired halls - substituted. A wireless was installed in 1921 and used in a "senior course in civics and current affairs". Throughout the period, King constantly urged the Governors, and through them the Committee and the County, for better facilities and a prompt start for the construction of the new school. He received few rewards for his pains. When in February 1921 he indented for a new carpet in his study he was instructed to buy a rug to cover the holes! His salary (£500 per annum) remained static, whilst those of other masters were increased.

Nevertheless, for the first time, the school started to attain results of a higher calibre. Between 1922 and 1925 eighteen scholars, mainly non-fee paying, and from Walthamstow elementary schools, went on to higher education and six open scholarships were won. These are figures which later seemed normal. Then they were outstanding. Many of these leavers were aided by scholarships from the Monoux Foundation. This was the result of the Essex County Council's purchase, in 1920, of the school site, the purchase money, and other funds, being invested to provide leaving exhibitions for Walthamstow children at the Monoux and High Schools, together with two entrance scholarships. The purchase resulted in the Monoux school being henceforth fully maintained in status.