A Co Antrim school has become Northern Ireland's first grammar to be placed in the Department of Education's formal intervention process.
Cambridge House, in Ballymena, has been criticised for the "inadequate" standards in its sixth form provision.
Although the latest damning Education and Training Inspectorate (ETI) report has not yet been made public, both the school and department have confirmed that Cambridge House is in formal intervention.
A Department of Education spokesperson said: "Sixth form provision at Cambridge House Grammar School was inspected recently and the overall quality of provision was found to be inadequate. The inspection report is due to be published shortly. The school has been placed in the formal intervention process.
"The North Eastern Education and Library Board (NEELB) is already working closely with the school to develop an action plan designed to address the areas for improvement identified during the recent inspection."
The ETI will continue to monitor the school's work to address the issues identified during the inspection.
Cambridge House has placed a message on the school's website stating it "welcomes the assessment, undertaken by the Education and Training Inspectorate, looking at our sixth form provision".
"The board of governors are committed to school improvement", it said, and added "since the time of the standard inspection in 2009, the percentage of pupils achieving seven or more GCSE grades at A*-C has risen from 76% to 82%, demonstrating improvement".
All parents will be notified in writing about the school being placed in formal intervention. However, some have already criticised the school for not informing them about the situation before it was made public. The statement from the board of governors added: "Our focus will therefore remain firmly on learning, teaching, raising standards in the school and providing the range of opportunities we offer to our young people."
In 2009 an inspection report into Cambridge House revealed that the percentage of pupils achieving seven or more GCSE grades at A*-C over the past three years, including English and mathematics, was well below the average for similar Northern Ireland selective schools. At GCSE grades A*-B, most subjects were more than 10 percentage points below the Northern Ireland average.
Cambridge House is the 27th school to be placed in formal intervention since the process began two years ago. Since then eight schools have exited formal intervention and one has closed. If a school has not exited within two years the department can restructure or replace its management team, merge or close the school.