Ysgol Coed Menai in Bangor was deemed to have a number of weaknesses of "a serious nature" by Estyn in February 2005, which included attendance rates of only 60 per cent at key stage 4 and the curriculum not meeting the requirements of the 1996 Education Act.
Estyn undertook a follow-up inspection in July this year. The school was found to have made progress in some areas, but Estyn said: "Because of the number of weaknesses and their serious nature, many of the improvements are still at an early stage of development. As a result, HM chief inspector of education and training in Wales has notified the Assembly government that this school will remain on the list of schools with serious weaknesses."
Ysgol Coed Menai is a special school for pupils aged between nine and 16 with emotional and behavioural difficulties.
In its new report, Estyn said that progress has been made in improving standards of achievement in Welsh first language, design and technology and in IT. Vocational provision for pupils at KS4 has been broadened and there is now more effective liaison with parents and carers.
But inspectors said that to maintain progress, the school should continue to improve the quality of teaching and further develop the curriculum so that it meets pupils' needs.
A Gwynedd council spokesperson said: "Gwynedd's schools service will be working closely with the school to ensure that these improvements continue."