The original Bute House, which belonged to the Marquis of Bute, stood on the site of the swimming pool. The house was first used as a school, albeit briefly, in 1918 by Belgian refugees. Four years later it became the boarding house for St Paul's Girls' School until the Second World War, when it was requisitioned by the army. On the opposite side of Brook Green, an independent girls' prep school was set up in 1932. Its Head, Miss F.A.Wigg, adopted the name Colet Girls' School during the war, when her school shared facilities with Colet Court and St Paul's Girls' School.

Our school bell has on it the initials "C.G.S." and is the last relic of that time. At the close of the war Miss Wigg was asked to set up the Junior School for St Paul's Girls' School in the building known as Bute House.  Misss Peggy Parry Okeden became the new Headmistress in 1945. Although successful, it was decided in 1955 that the school was to close because the old building was deemed dilapidated and unsafe. Parents raised money to build a new school on land leased to them by the Mercers' Company.

In October 1958 it was opened as an independent, mixed- ability preparatory school, called St Paul's Girls' Preparatory School, Bute House,  with Miss Peggy Parry Okeden continuing as its Head. The school from then had no formal connection with St Paul's Girls' School. The name was changed again to Bute House Preparatory School for Girls in the early 1990s as the second Head, Miss Jennifer Lee who had been Headmistress since 1973, retired and the current Head, Mrs Sallie Salvidant, joined the school. Bute House Preparatory School for Girls remains a highly successful, mixed-ability prep school that has cordial relations with its near neighbour, St Paul's Girls' School, but has no formal relationship with it. Children sit the competitive 11+ examinations for all senior schools on an equal basis with other 11- year-old candidates.