More secondary school pupils in Croydon study at underachieving schools than anywhere else in London, according schools inspector Ofsted.
Just 62 per cent of the borough's 11-18-year-old's attend a good or outstanding school, the education watchdog's annual report revealed.
Croydon also ranks second worst among the capital's 32 boroughs for the proportion of primary school pupils attending a school judged to be inadequate or in need of improvement.
Some 72 per cent of the borough's primary pupils go to good or outstanding schools, compared to a national average of 78 per cent.
Teachers blamed Croydon's post-riots reputation, the borough's diversity and funding inequality between areas of London for the poor performance.
Mike Dawson, secretary of the Croydon branch of teaching union NASUWT and a science teacher at Edenham High School, suggested the make-up of Croydon's population posed challenges for schools.
He said: "Croydon being the biggest borough in London, it has quite a wide, diverse area to accommodate.
"We have got high social deprivation in places like New Addington and Thornton Heath, where the pupils are obviously struggling to cope.
"We have high a immigration population as well. A lot of pupils coming in with English as an alternative language, which has an impact on their learning and if they don't make the progress Ofsted expects that means the school suffers.
"That doesn't mean to say the school is inadequate, it means they are doing the best they can with the pupils they have got and the teacher within those schools are doing a very difficult, challenging job."
Mr Dawson also suggested negative perceptions of the borough deterred some teachers.
He said: "The reputation of Croydon after the riots a couple of years ago plummeted a little bit. People looked at the word Croydon and thought 'hang on, I'm not sure I want to work there.' I'm sure that put people off."
"Difficult to attract best teachers to Croydon"
Gordon Smith, headteacher at Riddlesdown Collegiate and chair of the Croydon Secondary Headteachers group, said funding gap between outer and inner London boroughs meant boroughs on the fringes of the capital were disadvantaged.
Teachers at schools at inner London areas can earn up to £2,000 a year more than those working in outer London, although the rise of academies and free schools means rules are being loosened.
Mr Smith said: "It has at times been difficult to attract the very best teachers to Croydon as it has been possible for teachers to earn greater salaries by working a few miles away in inner London.
"We have been able to counter this at Riddlesdown by always being prepared to pay more to get the best.
"Now that the government has relaxed the regulations on teachers' pay, all schools, admittedly within tight budgets, now have much greater flexibility on pay issues and can compete for the best teachers on a level playing field."
Mr Dawson added: "The fact that we border inner London boroughs is part of the problem. As a teacher, you only have to go a couple of miles up the road to Merton or Lambeth to get an extra £1,000 on your salary.
"People are attracted by money in these economic times and teachers are no different. So there are issues attracting teachers.
"If funding was fairer across boroughs then I'm sure Croydon would improve. I know some schools that are struggling to fill vacancies."
Council: Schools are improving
Croydon Council said Ofsted's figures - collated at the end of the last school year in August - did not take into account recent improvement at some local schools.
The report includes the results of the most recent inspection at each school, meaning some ratings are years old.
A council spokesman said: "These tables don't reflect the most recent inspection results and we’re pleased to be able to say Croydon’s schools are continuing to improve and our GCSE results are significantly better than the national average.
"Since the figures in this report were compiled we’ve seen several more schools rated as good or better, meaning we now meet the national average of 78 per cent with this rating.
"Croydon Council and its schools are aspirational and we want all our children and young people to achieve their full potential. The council continues to both support and challenge those schools which need to improve to do so as quickly as possible."
The majority of London boroughs far exceed the national average proportion of children in good or outstanding schools, with several - including Camden, Hounslow and Islington - achieving a 100 per cent rate for secondary school pupils.
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