BUILTH Wells High School has become the third secondary in Brecon and Radnorshire to be placed in special measures.
The 640 pupil school has been branded "unsatisfactory" by inspection body Estyn, which will now monitor the school’s performance every term.
The College Road school follows Brecon High School and Llandrindod Wells High School in being judged as failing to meet expected standards and placed in special measures since 2014.
However inspectors said acting headteacher Ionwen Spowage, who has been in charge since April this year, hasn’t had enough time to make the improvements required.
The report found the school’s current performance and its prospects for improvement are unsatisfactory.
Among the shortcomings identified were a decline in standards in the crucial key indicators for key stage 4 GCSE year groups, years 10 and 11, for the past four years.
The report said: "Over the past four years, there has been a decline in standards at key stage 4. Performance in the most important indicators has placed the school consistently in the bottom 25% of similar schools based on levels of eligibility for free school meals.
"Pupils do not make the expected progress from previous key stages across many important indicators."
Around 7% of pupils at Builth are eligible for free school meals, against the Welsh average of 17.4%. But the proportion of pupils achieving five GCSEs at grades A* to C including English or Welsh and mathematics has been "consistently well below" expectations based on eligibility for free school dinners for the past three years.
The percentage of pupils gaining top A* to A grades at GCSEs has also declined over the past four years and is below the average of similar schools.
Performance for pupils in years 7 to 9, aged 11 to 13, has also "fluctuated" over the past four years and has been below expectations. Pupils are also failing to make progress as expected as they move through the school.
Standards in GCSE Welsh first language are solid, but boys’ achievement is "well below" that of girls - while boys do not perform as well as girls across GCSE classes.
In two of the last five years Welsh language pupils have helped the school place in the upper 50% of similar schools based on percentage of pupils gaining the key GCSE grades but there is a three year downward trend in achievement in Welsh language in years 7 to 9.
Inspectors also noted "nearly all pupils feel safe in school and consider that the school deals well with the few incidents of bullying". However while most pupils behave well inspectors found "poor behaviour" limited progress in some lessons.
Other failings identified included shortcomings in teaching and assessment in around half of lessons and pupils lacking confidence to work independently.
Inspectors said Builth’s prospects for improvement are unsatisfactory because Ms Spowage and her senior leadership haven’t had enough time to make improvements and the school has a poor record in addressing recommendations from previous inspections.
Senior leaders have also not had enough time to improve teaching and assessment due to their own teaching commitments.
The inspectors have made seven recommendations, which include raising standards at all key stages, improving the quality and consistency of teaching and assessment and ensuring line management effectively challenges underperformance.
Improving the behaviour of pupils who disrupt classes for others is also one of the recommendations.
Builth High must now draw up an action plan to show how it will address the recommendations.