A grammar school has been given the lowest possible Ofsted rating after its policy of teaching pupils in single-sex lessons was deemed to be "unlawful discrimination”.

Inspectors branded Chatham & Clarendon Grammar School in Ramsgate “inadequate” after they found boys and girls were not being given equal opportunities in the classroom.

But head teacher Debra Liddicoat has hit back at Ofsted, remarking that its conclusions were “inconsistent, unfounded and show a lack of understanding”.

Leaders at the grammar have been involved in a lengthy legal process to try to get the report - the worst in the school’s history - changed to make it a “fair reflection” of what inspectors saw. However, they have been unsuccessful in their efforts.

Ofsted officials wrote: “Pupils separated into single-sex classes are sometimes taught the curriculum in different ways depending on their sex.

“This included topics studied in the history curriculum or text choices in English.

“In another example, pupils told inspectors that girls must perform in an annual dance competition and boys do not have to.”

The difference in approach taken with the two groups, says Ofsted, amounts to “unlawful discrimination on the grounds of sex” under the Equality Act 2010.

In a statement released today, Ms Liddicoat highlights positives in the report, including good support for children with special educational needs and/or disabilities, pupils’ good behaviour, and teachers’ excellent communication skills and subject knowledge.

“These comments are brought to life by our recent examination results where we saw the number of A* and As at A Level increase by nearly 50% and nearly half of all GCSEs sat secured a 7 or above,” said Ms Liddicoat.

The school - previously rated ‘good’ - was founded in 2013 by the merger of the boys-only Chatham House Grammar School and girls-only Clarendon House Grammar School.

Boys and girls at the Chatham Street site are taught in single-sex classrooms in Years Seven to Nine, together for all but the ‘core subjects’ for Years 10 and 11 and attend all coed lessons in sixth form.

But during a two-day visit in March, Ofsted concluded that this separation was detrimental to pupils’ education and had unfavourable social effects.

“While most pupils are happy at school, some pupils are unhappy with the opportunities they feel they miss out on due to the practices of separating pupils by sex,” continued the report.

“Some pupils reported to inspectors that they become anxious when they enter mixed classes in some lessons in the upper school.

“This degree of separation and unfavourable treatment also affects whom they socialise with outside of lessons.”

But Ms Liddicoat argues that Chatham & Clarendon is far from the only Kent school to use this structure.

“Other similar schools have been inspected by Ofsted and the Independent Schools Inspectorate and it was not raised as an issue,” the head teacher said.

“In fact, we have also been inspected on several occasions since the Equality Act came into force – and inspectors have previously said that our approach ‘promotes equality of opportunity very effectively’.”

Ofsted found that despite the differences caused by separation by sex, the school provides an ambitious and broad curriculum as well as an array of clubs and other extra-curricular activities.

However, inspectors also raised safeguarding concerns, finding poor recruitment practices and record-keeping have put pupils at risk of harm.

“The school does not know whether necessary checks have taken place when staff have joined the school,” added Ofsted.

“Despite the school being aware of these problems for a number of years, steps have not been taken to address them.

“The school has not fully appreciated the importance of these records in keeping pupils safe.”

After the two-day visit in March, the school watchdog graded the Chatham Street provider positively in four of the five judgement areas, with just leadership and management rated poorly.

But following a legal battle to try and get the report changed, the school saw its mark for personal development, on review, downgraded from ‘good’ to ‘inadequate’.

Commenting on the setback, the school’s chair of trustees, John Waker, said: “We have continually fought for our school and our community, and have followed every avenue that is available to try and ensure that a fairer report is available for parents.

“However, at each stage we have been met with a lack of willingness to review evidence and positively engage with us.

“We will continue to take our battle forward, but now we need to get back to what matters most – the education and wellbeing of our students.”

KentOnline understands the school’s overall ‘inadequate’ rating is likely to trigger a department for education process known as a ‘termination warning notice’.

This means if the school does not take action to address the issues raised, it may be required to join a multi-academy trust.

“We have already developed an action plan and have been working through this while the complaints process has been ongoing,” added Mr Waker.

“Where appropriate, changes have been made and we will always look at how best we can enhance our school.”

Mr Walker says the school welcomes the recent move to scrap one-word Ofsted judgements and changes to the inspectorate’s complaints process but lamented that they were “too little, too late” for Chatham & Clarendon.

When approached for comment regarding the school’s claims, an Ofsted spokesperson said: “We don’t comment on individual inspections – we let our reports speak for themselves.”

A spokesperson for Chatham & Clarendon Grammar School added: “When the girls and boys schools merged in 2013, the decision was made to keep learning separate for the young year groups – but increasingly bring students together during their time with us.

“All students also share extra-curricular and enrichment activities.

“While we acknowledge that there are various views on single sex education, there is evidence that there are some real benefits.

“Furthermore, the feedback from our community and our academic performance show that the approach is one that has worked for us.

“In fact, over the last ten years, we have only had one parent who has asked us to have broader conversation with our community about our class structure.

“While we don’t agree with Ofsted’s interpretation of our version of the diamond model, we do think it is right to use this as an opportunity to explore our structure again.

“Our plan is to start a formal review of our single sex teaching, and this will include a broad and open discussion with students, parents and staff.

“This will happen over the course of this academic year.”