The state school Prime Minister David Cameron chose to send his daughter is being investigated for allegedly asking parents for money when offering places.
Grey Coat Hospital school in Westminster is being accused of sending letters to parents asking for £96 after their child has joined and a further £120 when they receive a sixth form place.
Asking for financial donations or contributions from parents is strictly forbidden under the school's admissions code for England, even if the money is refundable or given voluntarily.
The letter sent out to parents does not give an option to opt-out of the payment and simply states they should enclose a cheque, reported School Week.
The letter said the money is to fund extracurricular activities. Deputy head Elizabeth Stevenson wrote: "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.
"School Fund enables the school to provide many of the extracurricular 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."
The prime minister's daughter, Nancy, is due to start at the school less than a mile from 10 Downing Street in September. Former education secretary Michael Gove's daughter is also already a pupil at the school.
It is reported Gove was not asked, nor did he pay, any donation when his daughter was offered a place at the school. Cameron has refused to comment.
Cameron is the first Conservative prime minister to send a child to a state secondary school.
Grey Coat is one of the capital's most sought after state schools and gets more than six applicants for every place. Pupils at the school can study ancient Greek, Mandarin and Japanese, according to School Week.
A spokesman for the Department for Education said: "Any claim that the school admissions code has been breached will be investigated.
"We take any allegations very seriously."
IB Times UK is awaiting comment Grey Coat Hospital school.