BEVERLEY High School has lost its "outstanding" status after being told to improve by inspectors from Ofsted.
The girls' secondary – for many years a top performer in the East Riding – was already reeling after last year's GCSE performance was among the county's worst.
A national row over English exam grades was blamed for a dramatic dip in results.
Now, inspectors have ordered the school to improve its overall effectiveness – as well as the quality of teaching, achievement of pupils, leadership and management.
The behaviour and the safety of pupils was rated as good.
The improvement call is a blow for the 873-pupil school, which had been one of only two "outstanding" secondaries in the East Riding.
Headteacher Sharon Japp said: "The GCSE results last summer have made this inspection the toughest in the school's history.
"I am very disappointed by the outcome of the inspection but I am determined to address the issues raised by Ofsted as a matter of urgency.
"The inspectors observed almost two thirds good or better lessons over the two days, but this was not enough to meet the tough new expectations for a good or outstanding school."
Ofsted found teaching was too variable and students did not always make enough progress in all of their subjects.
Sixth form achievement also required improvement.
Ms Japp, whom Ofsted said "has a very clear view of how successful the school can be", was confident the secondary would soon be back on track.
She insisted: "The staff are ready to rise to the challenge that last year's exams and this inspection has brought and there will be a number of changes as we start to work on our new school development plan."
Ofsted noted governors held the head to account for performance.
Chairman of governors John Dunning said: "This has been a difficult year but by the end of the processes now being put in place, I am confident the school will have regained its position as one of the best in the East Riding."
Strengths highlighted by Ofsted included good or outstanding teaching in some lessons and good behaviour by pupils who enjoy school.
Strong systems were in place for student welfare and safeguarding.
Alison Michalska, East Riding Council's director of children, family and adult services, said: "Beverley High School was one of many schools nationally that suffered last summer as a result of the changes to the English grade boundaries.
"Despite this, the school continues to have excellent results in other subjects and many girls who attend Beverley High School do extremely well in their exams and go on to be high achievers.
"While Ofsted's report highlighted a number of areas for improvement, we are confident that with the support of the local authority's school improvement service, Beverley High School will once again be one of the East Riding's top schools."
