From Longman Dictionary of Contemporary Englishupper schoolˈupper school noun [countable] SESthe classes of a school in Britain that are for older students, usually aged 14 to 18
Examples from the Corpus
upper school• Pupils will be taught in the upper school, across the valley, until governors get the council's full report.• The upper school were remaining but it would be at least another week before he'd have to return. 