Table 2. Characteristics of included studies.

Author, year and location	Research design	N	Peer leader ages/ characteristics	Intervention type	Intervention characteristics	Frequency/duration of program	Leader selection process	School type	Training	Leaders: Outcome measures and findings	Recipients: Outcome measures and findings
Bausano (2006), USA (Dissertation)	Pre-post	110 mentors	Ages 13–18	Peer support	’Natural Helpers’ program. Students identified as existing ’natural helpers’ within the school given training to enhance skills.	~ 6 months. Unstructured meetings	Anonymous peer nominations	Secondary	Practice basic counselling skills, e.g., validation of emotions, problem-solving skills. Emphasis on positive coping strategies. Where/when/how to refer on.	Positive and Negative Affectivity Scale: no significant effect.	Not evaluated
Ellis (2004), Australia (Dissertation)	Quasi-experimental	99 mentors	Ages 15–17	Peer support	Peer leaders co-facilitate sessions with a supervising teacher. Each group contains 8–10 year 7 students and 2 peer leaders. Aims to help develop positive attitude and resilience.	12 x 45-minute weekly peer support sessions for 12 weeks	School-set criteria and answers to questions	Secondary	Intensive 2-day training. Emphasis on: creating a supportive atmosphere; active listening; leadership skills. Focusing on identifying needs and empathising with issues, instruction giving, planning a session.	SDQII-S and ROPE: non-significant improvement to self-confidence	Not evaluated
Ellis et al. (2009), Australia	Quasi-experimental	452 mentees	Mentees: ages 12–13; Mentors: ages 15–16	Peer support	Peer leaders co-facilitate sessions with a supervising teacher. Each group contains 8–10 year 7 students and 2 peer leaders. Aims to help develop positive attitude and resilience.	12 x 45-minute weekly peer support sessions for 12 weeks	School-set criteria and answers to questions	Secondary	Intensive 2-day training. Emphasis on: creating a supportive atmosphere; active listening; leadership skills. Focusing on identifying needs and empathising with issues, instruction giving, planning a session.	Not evaluated	SDQII-S and ROPE: no significant immediate effect on self-esteem. Significant f/u effect on self-confidence.
Froh (2004), USA (Dissertation)	Quasi-experimental	58 mentors	Ages 11–18	Peer support	’Natural Helpers’ program. Students identified as existing ’natural helpers’ within the school are given training to enhance helping skills.	Program running for > 4 years prior to study. Unstructured meetings	School-wide peer-nomination process	Secondary	3-day retreat. Positive self-coping strategies. Effective interventions, e.g. facilitative listening.	Personal Development Inventory: non-significant increase in measure that includes self-confidence	Not evaluated
Mason-Jones et al. (2013), South Africa	RCT	203 peer educators	~16 years old	Sexual health and wellbeing education	N/A–training program for peer educators only.	N/A	Volunteers. Additional selection criteria unclear	Secondary	2 x 1-hour skills sessions per month for duration of intervention. Delivered by non-profit organisations. Talk groups and mentoring sessions. 3-day camps–information sessions. Focused on developing range of psychosocial skills in the peer leaders	Self Esteem Questionnaire: non-significant improvements to self-esteem	Not evaluated
Roach (2014), England	Quasi-experimental	372 mentees	Mentees: ages 9–12 Mentors: described as ‘older’ but specific ages not given	Mentoring & anti-bullying	Mentors aimed to establish supportive mentoring relationships. Drop-in service, one-to-one mentoring and group mentoring.	Ranged from 1–30 sessions	N/A	Primary and secondary	Training varied across schools	Not evaluated	Students Life Satisfaction Scale (modified): Mentees: higher levels of life satisfaction after 1 year. Girls scored lower than boys. Older mentees: more likely to have lower life satisfaction 1 year post intervention. Mentees who had fewer meetings recorded higher life satisfaction scores at 1-year follow-up.
Sebire et al. (2018), UK	Cluster RCT	249 supported peers	All participants female and aged 12–13 years old	Physical activity (PA) promotion	‘PLAN-A’ intervention (Peer-Led physical Activity iNtervention).	10-week period of peer leaders asked to informally diffuse messages about importance of PA to friends and support to maintain/increase PA levels.	‘Influential’ girls in year 8 nominated by same-year peers. Top 18% of nominations were invited to train as peer supporters.	Secondary	Initial 2-day course, followed by 1-day top-up around 5 weeks later. Training took place off-site at various locations, depending on school. Content involved education about importance of PA; empowerment; development of personal skills; different methods of support; core objectives of program.	Not evaluated	Health-related quality of life (EQ-5D-Y): very small effect size in intervention group between T1 (post-study) and T2 (follow-up) (Cohen’s d = 0.088)
Shah et al. (2001), Australia	Cluster RCT	Group 1: 66 mentees Group 2: 47 mentees*(total = 113)	Group 1: Mean age 15.5 Group 2: Mean age 12.5	Physical health education	’Triple A’ asthma management programme. Mentors used games, videos, worksheets and discussions to teach group 1. Group 1 gave interactive (short acts, drama and song) performances to group 2 on asthma management.	Mentors prepare 3x 45 minute lessons. Younger students prepare presentation	Students in older year group (age 16–17) volunteered as mentors for group 1; all group 1 students recruited to act as mentors to group 2	Secondary	Workshop for older mentors (age 16–17 (‘asthma peer leaders’), how to use interactive delivery methods to teach peers	Not evaluated	Paediatric Asthma Quality of Life Questionnaire: group 1 and 2 completed questionnaire. Non-significant improvement in overall quality of life in both groups. Males showed significant improvement (p = 0.02; 95% CI) in emotional domain.
Song et al. (2018), China	Quasi-experimental	118 mentors; 124 mentees	Ages 12–14	Academic tutoring	Students divided by gender then split into highest and lowest performing halves of class. Required to study together for 30 minutes per day, four times per week. ~ 1 semester.	Range	Highest performing students in class	Secondary	?	100-item ‘Mental Health Questionnaire’: sig. increase in levels of guilt; sig. decreased social stress	100-item ‘Mental Health Questionnaire’: sig. decrease in mental health scores; sig. increase in ‘learning stress’
Wyman et al. (2010), USA	RCT	268 mentors	Mean age = 15.7	Suicide prevention	’Sources of Strength’ suicide prevention programme. Peers engaged in schoolwide messaging through presentations, public service announcements, videos, text messages and social network sites. Engaged trusted adults and encouraged friends to do same	Range as multiple tools employed	Peer and teacher nominations. Peers chosen to reflect diverse range of friendship groups	Secondary	Four hours. Led by certified trainers–focusing on coping skills and available resources. Interactive training.	Questions on recent suicidal ideation: decrease in suicidality over 3 months but not significant between conditions	Not evaluated
Yogev and Ronen (1982), Israel	Quasi-experimental	73 mentors	16 years old	Academic tutoring	’Young Tutors’ peer tutoring programme. Took place in schools and municipal youth clubs. Met 2 x per week during school hours.	?	Offered as elective subject to freshman students (aged 16 years)	Secondary	3 hours per week incorporated into class time. Lectures by school counsellors and invited specialists on adolescence and teaching methods. Included role-play, modelling and case analyses.	Self-Esteem Scale: significant increase in mentors’ self-esteem vs. comparison group	Not evaluated

‘?’ represents not enough information provided in the text. Version of ‘peer leader’ kept as originally stated in study.

*This study used a cascading approach, whereby one year group delivered an intervention to a younger year group (group 1), who then passed aspects of that on to an even younger group (group 2).

**Also included small number of Children and Young People’s Community Organisations (CYPCOs).