This paper aims to define the knowledge base of teaching in linguistically diverse secondary schools in England. Based on extensive interviews with the teachers across two schools, the paper identifies a range of good practices centred on flexibility and differentiation. These include diversifying teaching resources by using bilingual materials and dialogic tasks, as well as making adjustments to teaching by simplifying input and including cultural references. These practices are characterised by "a situated child-centred approach" which is underpinned by 10 core principles of multilingual classroom pedagogy for English as an additional language. Implications for education policy and practice are also discussed.