Multilingualism is highly prevalent in schools around the world. Yet, the relationship between multilingualism and academic attainment is not well understood. Where research on this topic exists, it has predominantly focused on how home language background impacts on academic success, lacking in a broader view of multilingualism which extends beyond home languages and also considers the identity component of being multilingual. This paper explores the relationship between multilingualism, multilingual identity and academic attainment. Using school-reported and student-reported data from five secondary schools in Southeast England, we disentangle the complexity of multilingualism in schools by contrasting self- and other-ascriptions of multilingualism, as well as investigating indicators of "multilingualism" and "multilingual identity." Our findings reveal meaningful differences between how students are identified and identify themselves as "multilingual," and foregrounds "multilingual identity" as a potentially more meaningful indicator for understanding academic attainment than multilingualism indicators traditionally used in United Kingdom schools.