This study will evaluate an intervention which bases social workers in schools (SWIS) with the aim they work more effectively with education colleagues and with children and families. The research design is a pragmatic cluster randomised controlled trial (RCT) and it builds on three smaller feasibility studies which found SWIS to be a promising approach and recommended it is explored further. Social workers will work within schools across 21 Local Authorities (LAs) in England, and the study will evaluate the impact of the programme by comparing outcomes between schools that have a social worker and those that continue as normal, without a social worker based on the premises. Schools will be selected randomly from a pool of schools put forward by LAs to receive a social worker, so that we can be confident any differences we observe are due to the intervention and not another difference between the groups. The primary outcome we are testing will be Child Protection (Section 47) enquiries, but we will also analyse other social care and educational outcomes to see what impact the intervention has on these. The study also includes an economic evaluation, which will calculate the costs of SWIS, and an implementation and process component which will explore how and why the intervention works as it does. Interim findings are expected in August 2021 and a final report will be published in August 2022.