THE borough's GCSE results appear to have fallen for the first time in 11 years.
Figures suggest the percentage of pupils in Croydon meeting the minimum expected standard fell from 64.4 per cent in 2013 to 62 per cent this summer.
Alisa Flemming, the council's schools chief, said it was too early to draw conclusions because the data has yet to be fully "validated".
The results would not normally have been released until the league tables are published by the Department for Education (DfE) in early 2015, though individual schools do choose to reveal their scores.
When the Advertiser requested the data in August the council said schools had released their results on the proviso they were not given to the press or made public.
A council officer, however, provided them to Mario Creatura, a Coulsdon West councillor, and they were then published in Gavin Barwell's manifesto, for whom he works as a researcher. Cllr Creatura said he did not explain how he was going to use the information.
Cllr Flemming, cabinet member for children, families and learning, has asked Mr Barwell to amend the online version of his manifesto because certain results, which had been subject to appeals or remarking, had changed since it was published.
"The main thing for me is the figures are not validated," she said.
"They are already inaccurate, as new data has come in from several of the schools. We've asked Gavin to update the online version of the document. It's the reason we don't formally publish the figures ourselves.
"What I would say is Croydon hasn't done any worse in comparison to other boroughs in London in that [the DfE] has changed the way the papers are marked."
Exam boards warned of "volatility" in this summer's results due to changes in the exam system designed to discourage schools from allowing their pupils to take their exams early. Opinion in Croydon appears split as to the impact of those reforms.
Riddlesdown Collegiate, in Purley, was one of just six schools to post improved results, according to the table published in the Conservative manifesto. Some 74 per cent of pupils achieved at least five A* to C grades including English and maths - the benchmark figure - up seven per cent from 2013.
Principal Gordon Smith said the changes to the exam system "played to the school's strengths".
"We're not a school that does early entries or re-enters students time and time again," he said.
"Our students and our teachers are used to sitting the exam at the end of the year and getting it right.
"I think removal of early entries will be for the good of students in the long term."
Edenham High's results, according to the unvalidated data, dropped 12 per cent this year, with fewer than half of pupils leaving school with at least five "good" GCSEs.
Head teacher Richard Rudd said he was "disappointed" but that, due to the re-designed exams, it was not possible to compare results with previous years.
This message was echoed by Margaret Mulchrone, head teacher of Thomas More Catholic School, in Purley, where the number of students meeting the minimum standard fell 16 per cent.
She said pupils had done "extremely well" considering they had to complete 25 exams at the end of the year, instead of a more "modular" system.
Cllr Flemming defended the borough's results but said she is concerned that fewer pupils are taking vocational subjects.
"Although we did well it almost feels there has been an undervaluing of the vocational courses," she said.
"I think we're in danger of moving into a situation where you have more of one and less of another. I think there's space for both."
Mixed results for Harris Federation schools
HARRIS Academy South Norwood is one of the schools where results have changed since they were published in the Conservative manifesto.
A spokeswoman said remarked exam papers would eventually mean 67 per cent of pupils achieved five A* to C grades in their GCSEs, down 13 per cent from 2013.
Principal Sam Hainey told the Advertiser: "Although we dipped in the benchmark results, our maths and English results individually were excellent.
"What happened was, for the first time, we had some students who got maths but not English, and others who got English but did not get maths. That's what caused it."
Harris pointed out that the number of pupils achieving the English Baccalaureate, a more stringent performance measure based on the traditional subjects, had risen from 40 per cent to 55 per cent.
Mr Hainey is also head at Harris Academy Upper Norwood, which is in the process of merging with its South Norwood counterpart.
It appears to have posted the most-improved results in Croydon, after its previous iteration - Westwood Girls' College - finished bottom of the league table in 2013.
Mr Hainey said he was particularly pleased with attainment in English and maths.
"We had to come in and restructure both subjects in order to raise standards," he explained.
"We had to convince the children they can do well. Because of weaker teaching in the past we had quite a lot of work to do to make up for lost ground. It's been a huge success."
