Table 1.

STASH study criteria for Progression to stage III Randomised Controlled Trial

	Criteria	INDICATOR and TARGET	Recommendation if GREEN, AMBER or RED target met	METHOD OF ASSESSMENT	RATIONALE
1	Was it feasible to implement STASH in 4 of 6 schools?	GREEN: In each of 4 schools, 60% of nominated students are recruited and complete the training	Very strong indication to proceed	Project monitoring data	Based on learning from ASSIST, 60% is estimated as the proportion required to ensure that peer supporters are representative and reach across the entire year group.
		AMBER: In each of 4 schools, 50% of nominated students are recruited and complete the training	Medium indication to proceed. Discuss with Trial Steering committee (TSC) and proceed with identified plan to improve performance on indicator in Phase III trial
		RED: Amber target achieved in fewer than 4 schools	Indication of doubt as to whether to proceed. Discuss with TSC, and only proceed if other indicators are amber/green and there is a clear mitigating strategy
2	Was STASH acceptable to peer supporters in 4 of 6 schools?	GREEN: In each of 4 schools 60% of peer supporters who complete the training, send three or more messages/have three or more conversations, and attend two or more follow-up meetings and 60% of peer supporters report that they ‘liked’ the role	Very strong indication to proceed	Facebook monitoring data Peer Supporter Questionnaire	We consider 60% a reasonable target given the sensitivity of the topic and challenge involved for peer supporters. 60% represents a majority while not providing an over-ambitious target, given that the intervention is new to schools and not institutionally embedded. We would expect role acceptability to increase with further iterations (e.g. in a full RCT) which lead to greater clarity and institutional support. We view 60% as a ‘starting point’ for this feasibility stage.
		AMBER: In each of 4 schools 50% who complete the training send three or more messages/have three or more conversations, and attend two or more follow-up meetings and 45% like role	Medium indication to proceed. Recommend as per amber target for Criteria 1
		RED: Amber target achieved in fewer than 4 schools	Indication of doubt as to whether to proceed. Recommend as per red target for Criteria 1
3	Was STASH acceptable to stakeholders and target group?	GREEN: In each of 4 schools, 60% of students who are exposed to STASH agree that the intervention was acceptable. No major acceptability issues raised by participating schools (identified via process evaluation or communication with school) Less than 15% of peer supporters report that parents were unhappy about them being a peer supporter	Very strong indication to proceed	Follow-up Questionnaire Process evaluation interviews Peer Supporter questionnaire	We consider 60% a reasonable target given the sensitivity of the topic. 60% represents a majority and is realistic in the context of a feasibility study. Acceptability to teachers and school leadership will be assessed qualitatively, hence a focus on identification of major issues rather than a quantitative target.
		AMBER: In each of 4 schools, 50% rate intervention as acceptable Less than 20% of peer supporters report that parents were unhappy about them being a peer supporter One or two major acceptability issues raised by participating schools but mitigating strategy identified	Medium indication to proceed. Recommendation as per amber target for Criteria 1
		RED: Amber target achieved in fewer than 4 schools Major acceptability issues raised by schools with no possible mitigating strategy	Indication of doubt as to whether to proceed. Recommendation as per red target for Criteria 1
4	Were the evaluation methods acceptable and feasible?	GREEN: In each of 4 schools, student response rates of >70% at baseline and follow up	Very strong indication to proceed	Baseline and Follow-up Questionnaires	We consider a response rate of 70% sufficient to undertake analysis, and feasible given that this cohort are undertaking public examinations at the end of the year. Response rates in the pilot were lower than expected for a school survey. Parental opt out has been very low and nearly all students in attendance complete the questionnaire, but due to the age group (and linked to area deprivation) there are students who are regularly absent (e.g. because they also attend other services).
		AMBER: In each of 4 schools, student response rates of >60%	Medium indication to proceed. Recommendation as per amber target for Criteria 1
		RED: Amber target achieved in fewer than 4 schools	Indication of doubt as to whether to proceed. Recommendation as per red target for Criteria 1