Grey Coat Hospital School in central London sent a letter to parents offering a place and asked them to return confirmation along with £120 for the School Fund.
The School Fund is used for extra-curricular activities and the note adds that paying the cash was "an important way" of showing commitment to the school.
"If you confirm your offer of a place at Grey Coat for September 2015, then we ask you to pay School Fund, which for sixth form students is set at £120 per annum," the letter said.
"School Fund enables to school to provide many of the extra-curricular activities that are run for Year 12 throughout the academic year, in particular the team-building activities/resources which are provided during induction week."
It adds: "Paying School Fund is an important way of showing us that you are serious about taking up a place with us in Year 12 in September."
School admission rules state that teachers cannot ask for any type of financial contribution as part of the admission process.
According to its website Grey Coat Hospital should 'enable students to take charge of their learning, make decisions based on Christian values, live in the world as independent women and men and meet the challenges of the twenty-first century'.
It was founded in 1698 and is a Church of England Academy for girls aged eleven to eighteen. It is one of the country's top performing state schools.
Last week the Prime Minister was told his daughter had been offered a place at the school in Westminster.
The request for money could now be referred to the Office of the Schools Adjudicator for investigation.
The school refused to comment.
A Department for Education spokesperson: "Any claim that the School Admissions Code has been breached will be investigated."