Ofsted found standards at Peckham academy, in south London, to be "exceptionally low", although there had been rapid improvements since the pound;26 million school opened in 2003.
Inspectors gave the school a satisfactory rating overall but said that its managers failed to address significant weaknesses in the sixth form quickly enough.
"Too many (sixth-form) students failed to make satisfactory progress during the first year," the report said.
The criticisms follow Ofsted's report last week ordering teaching to improve at the pound;32m City of London academy. Inspectors found too many pupils struggled with basic reading and writing.
Last month, the Business academy in Bexley, Kent, was criticised by Ofsted, while the Unity City academy in Middlesbrough was placed in special measures last year.
Peter Crook, principal at the Peckham academy, which is sponsored by Lord Harris, the carpet magnate, said the sixth form had only been open a year and there had been no A-level results to judge it on.