And last year, all Year 11 pupils passed at least five GCSEs at grades A*-C. However, subject inspection grades ranged from good (grade 2) to a poor grade 5 for history at Key Stage 4, indicating many important shortcomings.
Inspectors said lessons should be more exciting, with less direction by teachers. The school has yet to produce the action plan required in response to the inspection report, as it failed to meet some of the standards for registration.
Inspectors said it should do more to promote tolerance and harmony between different cultures, ensure its child protection policy is being fully practised, improve security and install sufficient washing facilities.
Andrew Tamplin, headteacher, said the school had been pleased with reports of its academic strength. He denied the school did not teach pupils enough about Wales or other religions and cultures, and was dedicated to teaching and promoting the spiritual development of its pupils through Christianity.
Being able to afford flashy equipment or bigger classrooms was not its biggest priority, he added.
The school, a registered charity based in a church and a chapel in Machen, keeps its fees low so that poorer parents who want a Christian education can afford it. However, this limits resources.
Pupils were found to be happy, polite and saw themselves as part of "one big family". But Estyn team leader Jeff Jones, also found them to have an under-developed knowledge of Welsh culture.
The school does not offer Welsh or follow Y Cwricwlwm Cymreig, the Wales-only curriculum. Religious education was based solely on Christian teachings.
The school opened in 1982 with just seven pupils and has expanded on to two sites. It makes the best use of "inadequate" accommodation, but classroom areas were said to be restrictive and cramped. The school's leaders are seeking alternative accommodation.
Inspectors found pupils displayed good logic, especially in maths and science, but had a less-developed ability to think creatively or independently and relied on teachers too much when answering questions.