In a report published this week, the Office for Standards in Education said that the teaching of the national curriculum at the Institute of Islamic Education, in West Yorkshire, was inadequately planned and pupils' ability was poorly assessed.
Inspectors said: "Methods employed, such as the memorisation of text, are better suited to the madrasah (Islamic) curriculum. Teachers show limited understanding of pupils' aptitudes, needs and prior attainments. Pupils'
books are rarely marked. This ... contributes to overall poor public examination results at the age of 16."
Last year, only 9 per cent of the 105 pupils at the pound;1,410-a-year boarding school gained at least five A*-C grades at GCSE.
The institute caters for boys who intend to be imams or Islamic scholars.
In the morning they have Islamic studies and in the afternoon they are taught national curriculum subjects by different instructors.
Inspectors, who visited the Dewsbury school last month, concluded that the emphasis on Islamic teaching came at the expense of secular studies.
They said not enough time was allotted to the national curriculum and teaching was often directed most towards the few more able pupils in the class.
Inspectors said that the poor quality of secular teaching led to lower standards of behaviour in the afternoon.
They also found that parts of the school building, including the dining-room and boarders' toilets, were untidy and dirty.
The Institute failed to meet Department for Education and Skills conditions for registration of private schools. It must now draw up an action plan for meeting these requirements, which will be monitored by the department.
But Muhammed Mulk, head of the school's secular section, blamed poor pupil motivation.
He said: "Parents send their children here for an Islamic education. They don't want their sons to take exams.
"Our teachers make an effort to tell them education is important. But the boys do not bring books or pens to lessons, and their attitude is very negative."
Earlier this year, chief inspector of schools David Bell criticised independent Muslim schools for failing to promote tolerance of other cultures.
David Bell profile 13 www.ofsted.gov.uk