Secondary school is defined in section 5 of the Education Act 1996.
(along with sections 2 to 4).
In summary, it is a school for providing one or more of the following:
• full-time education aimed at children of compulsory school age who are 10½ to 11 years old and are taught together with children aged 12 and above;
• full-time education aimed at children of compulsory school age who are at least 12 years old.
In some circumstances it can also include part-time education for pupils who are receiving the rest of their secondary education elsewhere, full-time education for pupils over compulsory school age but under 19, and education for pupils aged 19 or older who are finishing courses they started at the school before turning 19.