Joanne Harris, 57, joined Hemdean House School in Hemdean Road as a part-time physics teacher in January 1989.
was appointed deputy head in 1994 and in 2001 was made head — only the eighth in the independent school's 150 years — after the retirement of Pat Pethybridge.
, of Shiplake Cross, said: "Hemdean
is a wonderful place to work, with its caring ethos and family atmosphere — everyone is important to the community and is valued.
My first seven years as headteacher have been hard work but extremely happy and fulfilling and I look forward to future years."
Staff celebrated the anniversary with champagne and a cake decorated with a picture of the school at a teacher training day.
said: "I hope this will be followed by celebrations for the 150th anniversary of the school and the opening of the new school hall, which will be built during the course of this year."