Chatham Grammar School for Boys in Kent has been given the lowest possible rating by schools inspectors.
Following an Ofsted inspection in June, it received an inadequate grading, which places it in special measures.
Its leadership and management came in for criticism because achievement "had not improved quickly enough" since the previous inspection.
Headteacher David Marshall announced his retirement last week and has not returned for the new school year.
The school, which is an academy and not run by the local authority, is the second grammar school in the country to be judged as failing.
'Weak and inadequate'
Ofsted inspectors highlighted "weak and inadequate teaching" and said teachers did not "consistently plan lessons that challenge students and extend their thinking or give students detailed verbal and written feedback".
They also found the achievements of students were "not consistently good", and that leaders and governors did not check the progress of different groups of students accurately, while sixth form students' progress was "too variable across subjects".
Behaviour in lessons had also declined since the previous Ofsted inspection.
However, the report said students' behaviour around the school was good, and pupils with special educational needs made good progress because of "well-targeted support".
Interim executive principal Denise Shepherd and interim principal Stuart Gardner were asked by the chair of governors to move from their posts at the Rochester Grammar School, where they had the same roles, to run the school.
Ms Shepherd said some of her senior staff from Rochester would also assist colleagues at the Chatham school as they took the school forward "into a new era of achievement".
In a letter to parents she said the Ofsted report did not mean the school was failing.
"It has an academic record that many schools would be proud of, with 95% of pupils achieving five or more A*-C GCSEs including English and Mathematics.
"The inspectors' concerns were over the rate of improvement in some areas of development and that is why we have been asked by the trustees and chair of governors to work with the school," Ms Shepherd said.
"We have already started the work of rapidly improving the effectiveness of leadership and management."
She added that as part of the process of improvement, lessons and teacher involvement across the curriculum would be regularly monitored.
A Department for Education spokesperson said: "Under-performance in any type of school is unacceptable.
"An educational advisor will visit Chatham Grammar School to ensure that the academy is taking swift action to address the issues highlighted in the Ofsted report."