Alys Keith-Lucas started the independent preparatory school, which is on Dry Hill Park Road, in 1919 with three pupils – one of whom was her son Brian.
To mark the milestone her great granddaughter Sarah came to take part in the festivities accompanied by her aunt, Mary Benjamin.
A special service was held at St Saviour’s Church, with the congregation sharing memories – including those of two former pupils whose own children now attend the school.
Then Ms Keith-Lucas talked to the children and told them about her role as a BBC meteorologist and weather presenter.
Two former headmistresses, Jackie Bacon (1988-2005) and Susan Webb (2005-15), joined the present headmistress, Katy Joiner, to cut a special cake at the party the following weekend.
More than 500 people attended the event, around half of whom were former pupils and staff after an appeal the school launched last year.
The current pupils performed orchestral recitals, which included music written by one of the teachers, Carol Bottle, during the 1920s.
They also demonstrated different dances from down through the decades, as well as traditional maypole and country dances.
Mrs Joiner said: “We have had a wonderful week celebrating 100 years of our fantastic school and bringing the whole school community together.
“We were thrilled to welcome members of the Keith-Lucas family to share in the celebrations as well as so many former pupils, staff and trustees who enjoyed reuniting with old friends and taking a trip down memory lane.
“A wonderful time was had by one and all, and we are very grateful to everyone who contributed to making it such as success.”