School-based interventions for young people with emotional/mental health problems are often provided by external practitioners and their relationship with host schools is a key influence on implementation. Poor integration within school systems, schools' tendency to define interventions around pupils' behaviour and teachers' control over access, may undermine therapeutic relationships. This study examines how one school-based intervention--Bounceback--addressed these challenges. Methods comprised interviews with programme staff, school staff and service users. Bounceback sought to develop therapeutic relationships through creating a safe, welcoming place and maximising pupils' choice about how they engaged with it. To ensure Bounceback was delivered as intended, staff developed five conditions which schools were asked to meet: adhering to referral criteria, ensuring that attendance was voluntary, appropriate completion of referral forms, mechanisms for contacting pupils and private accommodation to maintain confidentiality. Pupils reported high levels of acceptability and described relationships of trust with Bounceback staff. Although pupils had choice about most aspects of Bounceback, teachers controlled access to it, partly in order to manage demand. The study highlights the need for external agencies to communicate their aims and needs clearly to schools and the importance of peripatetic practitioners being well integrated within their parent organisations.