North London Collegiate school (NLCS) in Edgware is facing investigation after all its A-level entries received A* or A, The Guardian has reported.

Senior leaders and teachers at the 22,000-a-year girls’ school are being investigated for alleged malpractice in setting teacher-assessed grades, which replaced formal exams cancelled by the government because of the Covid-19 pandemic.

The Guardian reported that more than 20 cases involving NLCS have been passed to malpractice committees convened by A-level examination boards, which held a series of confidential hearings this month.

A spokesperson for Pearson, the exam board involved in the investigations, said: “We take allegations of malpractice very seriously and investigate all those put to us. Where malpractice is found we have systems in place to ensure, as far as possible, that appropriate sanctions are taken according to the evidence. We cannot comment on individual cases.”

A spokesperson for NLCS said: “The process and findings of any such investigations are confidential between the exam board and the parties. However, we can confirm that there has been no finding of centre [school] malpractice by NLCS.”

Later the NLCS spokesperson later added: “We can confirm that the investigation has concluded.”

Bridget Phillipson, Labour’s shadow education secretary, called for a full investigation by the Department for Education and added: “These findings will do nothing to dampen suspicions that certain private schools were gaming the system during the suspension of exams due to Covid. It is time for the education secretary to launch her own investigation into the assessment of grades during Covid to establish just how widespread this exploitation was.

“Labour would end private schools’ tax breaks and use the money to invest in a brilliant state education for everyone.”

A spokesperson for Ofqual commented: “Ofqual takes allegations of malpractice extremely seriously and we require exam boards to investigate such allegations and address any proven cases of qualifications-related malpractice in a school or college. It is not appropriate for us to comment on specific cases.”