The attainment in national examinations and progress of pupils to the age of 16 in London is the highest in England. Nevertheless, there is still a significant number of 16- to 19-year-olds who are not in employment, education, or training (NEET). Those who are the most vulnerable to becoming NEET are the young people who have disengaged from mainstream education. This article draws on a comprehensive examination of the effectiveness of an alternative education provision (AEP) for pupils who were disengaged from mainstream schools in one London local authority. Through the application of Bronfenbrenner's ecosystems theory, the study explored the impact of different ecosystems on young people's disengagement. The findings in evaluation studies of other AEPs and the findings in this study indicate that AEPs – and the curriculum, pedagogy, and pastoral care that they offer – can, and do, make a considerable difference to the educational outcomes of disadvantaged children, as well as offering insights for mainstream education. Thus, the study contributes to the current debate on the organization and structure of the 14–19 education system in England under raising the participation age (RPA) to 18, the new legislation that came into force this academic year.