THREE-times outstanding Beverley Grammar School has lost its top rating after being ordered to improve by Ofsted.
The boys' school is the third East Riding secondary that has been told to improve by the watchdog in a fortnight.
There are now no outstanding secondaries in the East Riding, after Beverley High School was also stripped of its top rating this week.
On Tuesday, Ofsted launched a wave of inspections at 15 schools in the area to find out why so many are under-performing.
The downgrading is a blow for the boys' school, which was among the top 1 per cent of high-fliers nationally when it secured its third successive outstanding verdict at its last inspection five years ago.
Disappointed headteacher Grahame Hodson said: "The expectations Ofsted have of schools' performance are higher now than they have ever been and the inspection methodology is even more challenging and rigorous.
"Our attainment is consistently well above average and students make expected progress by the end of Year 11.
"Our A-level results were the best in the East Riding last year but we do appreciate the need to make progress in certain areas very quickly."
Mr Hodson said the school would work tenaciously to move as quickly as possible to good or outstanding in all areas inspected by Ofsted.
He said: "We will be using this setback as a driving force to improve our standards further.
"Many of the improvements required are already under way and this report will help us to accelerate their implementation."
Ofsted found the school requires improvement for overall effectiveness, achievement of pupils, quality of teaching, leadership and management. The behaviour and safety of pupils was rated as good.
Chairman of governors Chris Bodsworth said: "I am disappointed with the outcome but in the end it's a splash of water in your face, you take a deep breath and you get on with it.
"At the end of the process we will come out of it even stronger than we were previously."
He insisted parents of children at the 889-pupil school, which is the country's oldest secondary, should not be alarmed.
Mr Bodsworth said: "Nothing has gone wrong. Ofsted's expectations are much higher now and we are aware what needs to be done."
Ofsted said achievement must improve because not enough pupils exceed the progress expected.
The sixth form requires improvement because progress is not good.
Teaching must be improved as it does not promote good progress for pupils throughout the school.
Work is not consistently well planned to match students' capabilities, especially for average and less capable students.
Leadership and management at all levels requires improvement.
Ofsted also noted strengths and said standards are consistently well above average. Pupils working at higher levels achieve well.
The head and senior leaders are improving the quality of teaching.
