AbstractIntroduction and AimsSecondary school‐based counselling (SSBC) is used widely throughout the United Kingdom (UK) as an early mental health intervention, and clinical supervision is necessary for its safe and effective practice. Currently, there does not exist a body of research to inform best practice in this area of supervision. The purpose of this study was to explore the supervisory requirements of the experienced secondary school‐based counsellor and what best practice looks like.MethodThis study used situational analysis constructivist grounded theory methodology to explore and develop a grounded theory of best practice in supervising the experienced secondary school counsellor in the UK. The descriptions of best practice were examined from the interviews of 17 experienced secondary school counsellors and/or supervisors.FindingsFrom the data emerged one core category (‘a good fit’) which encapsulated four theoretical categories (‘the supervisory relationship’, ‘understanding the working context’, ‘safeguarding’ and ‘the shared supervisory journey’). The study indicates that the supervisor needs an understanding of the unique context of the secondary school as a therapist's working environment in order to support the supervisee with often complex work. There needs to be an awareness of how elements from within the school and/or counselling agency can impact on the supervision regarding boundaries, trust and containment so they do not impact the supervision in a detrimental way.ConclusionsSupervisor and supervisee need a shared understanding of the nature of supervision within this context for it to be a ‘good fit’ for both. This has implications for supervision training and the externality of supervision within this context.