Finchley strictly Orthodox school Pardes House Grammar has been ranked as good by inspectors.
Pupils "often reach standards that are at least one or two years ahead of their contemporaries," Ofsted reported.
Pardes House has 153 boys aged up to 16 on its roll. Pupils generally go on to study at yeshivah.
Students sit GCSEs early and nearly two-thirds last year achieved good grades in at least five subjects, including maths and English.
Many of those in year 11 - when students at most schools sit GCSE - gained two or more A-levels, performing particularly well in English, maths and biblical Hebrew.
"The most able students achieve well, with a number gaining an A-level qualification by the end of year 10," inspectors noted. "However, at both GCSE and A-level, across a range of subjects, they do not achieve as many of the high grades as they should."
Pardes House was complimented on its "impressive range" of secular subjects, including courses in music, Jewish calligraphy and web design. "These, together with trips and residential visits, ensure that students are well prepared for life in modern Britain."
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