WOODCOTE High has been criticised by the education watchdog which claims its sixth form is inadequate and there is not enough good teaching.
In its first inspection since becoming an academy last year, the school in Meadow Rise, Coulsdon, was rated as "requiring improvement" overall.
The new Ofsted ranking, replacing "satisfactory", is one lower than the "good" the school received in its previous inspection in 2008.
Inspectors highlighted maths and sixth-form teaching as areas needed improving, praising progress made by disabled and SEN students, among other matters including above average attendance.
Head teacher Mark Southworth said: "There are now some aspects that were highlighted by the new, much more rigorous, inspection framework which we now need to address and we are happy to do this.
"There is much positive in the report and it is good to have confirmation that our students behave well, feel safe, enjoy their learning and are well supported in lessons."
Lead inspector Robert Ridout said students' progress in the school's relatively new sixth form, which published its first results in 2011, was "well below expected," especially in A-level courses compared to BTECs.
Sixty-eight per cent of the school's Key Stage 5 students achieved three or more A-levels or equivalents at A*-E last year, 55 per cent without equivalents.
Mr Southworth rejected the inspector's assessment that sixth-form standards had slipped, saying "curriculum issues" were being resolved.
He said: "Of the 13 sixth-form lessons observed by Ofsted during the inspection, 10 were rated good or better. We very much believe that this is a curriculum issue which can easily be put right."
A new curriculum aiming for the "correct balance" between vocational and academic was showing results, he said, and more vocational courses will be offered from next year.
Mr Redout added that more challenging targets, among other things, were needed in maths teaching, which had slipped behind English.
Last year, 70 per cent of students in Key Stage 4 gained a grade A*-C in maths, compared to 91 per cent in English.
Mr Southworth said the school had responded to the gap after identifying it last August.
He said: "We responded last year by recruiting additional mathematics teachers to provide smaller classes and give targeted help to students.
"Ofsted accepted this but were unable to give us credit for these actions until the next results come out in the summer."
The inspector also criticised the school's now-ended policy of entering students early for maths and English GCSE exams, saying it "limits the attainment of the more-able students".
Mr Southworth said there would be no early entries in 2014, prompted by Ofsted's saying in September 2012 that it disapproved of the policy.
The percentage of the school's students getting good GCSEs remains well above national average.
Sixty-nine per cent achieved the benchmark of five good grades including maths and English, or equivalents, last year, compared to 59 per cent nationally.