OASIS Academy Coulsdon is appealing some of its GCSE results after receiving among the lowest overall scores in Croydon.
The school, in Homefield Road, had yet to reveal its benchmark figures, prompting concern among parents looking to make an informed choice of school preferences ahead of the deadline at the end of the month.
Oasis Community Learning told the Advertiser this week that 46 per cent of pupils left the school with at least five A* to C grades including English and maths, the government's main performance measure.
That is 15 per cent lower than in 2013 and the academy's lowest result since 2011. The Department for Education's (DfE) minimum requirement is 40 per cent.
A spokeswoman said the figure could change "significantly" pending a successful appeal of its English results, which saw 65 per cent achieve A* to C grades.
Barring a dramatic reversal, the school will be the fifth sponsored academy in Croydon to see results fall in 2014.
Last week the Advertiser reported results for the borough as a whole may have fallen for the first time in 11 years, from 64.4 per cent of pupils achieving at least five 'good' GCSEs in 2013 to 62 per cent this summer.
That conclusion is based on "unvalidated" data, with a number of schools, including Oasis Academy Coulsdon, questioning individual results.
The Harris Federation's academies in Crystal Palace and South Norwood both saw results fall, as did Quest Academy, in Selsdon, and Addington High, which was converted into an academy to improve standards but achieved a benchmark score of 37 per cent, the lowest in the borough.
While other types of schools - such as local authority-led Edenham High and voluntary aided St Mary's Catholic High - also posted worse results, it is the first time so many academies have failed to improve since the - at the time controversial - conversions in 2008.
Oasis Community Learning took over Coulsdon High in September 2008 and has gradually improved the school's results. Even if the school's appeal is unsuccessful, this year's score is still five per cent higher than it ever achieved as a local authority school.
A spokeswoman said: "Since opening in 2008, attainment levels at Oasis Academy Coulsdon have improved significantly and, in 2011, the academy received a 'good' judgement from Ofsted.
"All indications are that results will continue to improve and, as a result, local residents' confidence in the academy is returning."
She said the school would provide parents with the full GCSE results as soon as information is available.
Exam boards had warned this year's results could be "volatile" due to changes to the exam system designed to discourage schools from entering pupils early or on multiple occasions.