Three cluster randomised controlled trials in Portugal, Czechia and the United Kingdom to test the effectiveness and cost-effectiveness of an augmented social play intervention in schools to support real- world connection and sense of belonging (Lina trial protocol)

• Background: Mental ill-health and loneliness represent major challenges for today’s youth, exacerbated by unprecedented societal demands, including concurrent socialisation in physical and digital worlds. A strong sense of belonging can buffer against these challenges, and in adolescents is largely constituted in schools. Augmented Social Play (ASP) involves smartphone-led cooperative face-to-face gameplay which integrates immersive storytelling, augmented reality, and evidence-based psychology to strengthen belonging. Building on this foundation, Lina is a multi-session intervention designed for classroom-based delivery during routine school lessons. • Methods: Three parallel hybrid type II cluster pragmatic Randomised Controlled Trials will be conducted in secondary schools in Czechia, Portugal and the UK. The trials will examine the effectiveness of Lina - compared to standard lessons - in strengthening classroom belonging, reducing symptoms of mental ill-health and improving wellbeing, loneliness and stigma. 40 classes of students whose modal age is 12-13 years will participate in each country. Effectiveness will be examined on completion of Lina and after 6-months, using the student-report Delaware School Climate Survey, the Social Anxiety Scale for Adolescents, the Patient Health Questionnaire-8, the Social Awareness subscale of the Social-Emotional Learning Edition of the Strengths and Difficulties Inventory System, the WHO-5 Well-being Index, the UCLA Loneliness Scale and a tailored version of the Peer Mental Health Stigmatisation Scale-Revised. To examine cost-effectiveness, quality of life will be measured in a corresponding manner, using the Child Health Utility 9D and the EQ-5D-Y-3L, and analysed alongside bespoke resource-use inventories, completed via teachers and/or school records. An embedded developmental evaluation of implementation will draw on researcher observations of Lina , teacher-reported Feasibility, Acceptability and Appropriateness of Intervention questionnaires and teacher focus groups. A second developmental phase of implementation evaluation will follow the effectiveness trial, while control classes do Lina with their teachers. An integrated qualitative evaluation will examine students’ experiences of Lina using individual interviews and focus groups, including a particular focus on students with characteristics that put their sense of belonging at risk. • Discussion: Findings will speak to the potential impact of Lina on youth mental health. They will also inform Lina implementation guidance for diverse national and international school contexts. • Trial Registry Information: This trial is prospectively registered with ISRCTN (Identifier: ISRCTN11613092), registered on 15 September 2025, https://doi.org/10.1186/ISRCTN11613092