Immanuel College expects a full house in September with a record roll of 700 students, including for its preparatory school.
The fee-paying college in Bushey is set to welcome a full complement of 96 year-seven students this autumn.
Headmaster Gary Griffin said it was “gratifying that we are so popular and that I am in a position to expand the size of the school, though I have no wish to go beyond 700.
We are proud of our relatively small and family community, with small class sizes and friendly atmosphere.”
As well as plans to build new classrooms, the growing numbers have led to an expanded choice of A-level subjects with business studies, music and sociology available alongside more traditional academic choices. "PE, computer science, art and photography have also attracted record numbers this year," he said.
The school was bucking the national trend with a record number of pupils also opting for modern languages - Spanish, French and Ivrit - at GCSE and A-level.