Table 1 Description of outcome measures, assessed at baseline, 3 and 6 months among schoolchildren, the SELFCARE project

Demographic data

Data on age, sex, school, class, cohabitation and family socio-economic status will be collected as in the national Health Behaviour in School-aged Children survey

The Strengths and Difficulties Questionnaire (SDQ)-youth self-report [32]

The SDQ comprises 25 items describing psychological or behavioural attributes. The items have three response categories ‘‘very true’’, ‘‘somewhat true’’ or ‘‘not true’’. The instrument can generate five sub-scales scores (emotional problems, conduct problems, hyperactivity, peer problems, and prosocial behaviour). The total difficulties score is the sum of the subscale scores excluding the score of prosocial behaviour. The total difficulties score ranges from 0 to 40 with higher values indicating poorer behavioural and emotional functioning and well-being. The subscales ranges from 0 to 10 with higher scores indicating poorer functioning and well-being for four of the subscales (emotional, conduct, hyperactivity–inattention and peer problems) and better functioning and well-being for one of the subscales (prosocial). Goodmann et al. has shown a dose–response relationship between total difficulties scores and the risk of having or in a 3-year period developing a mental disorder [33]. The odds-ratio for having a mental disorder per one-point increase in the youth self-reported SDQ total difficulties score was 1.23 (95%CI 1.21 to 1.25). The odds-ratio for developing a mental disorder with-in a 3-year period per one-point increase in the youth self-reported SDQ total difficulties score was 1.16 (95% CI 1.13 to 1.18). These are results of a population-based observational study. However, Goodmann et al. argues that differences in mean total difficulties scores between intervention and control groups in experimental research can legitimately be interpreted as reflecting genuine differences in mental health [33]

Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS) -short version [37,38,39]

The seven-item WEMWBS scale is designed to capture a broad conception of well-being covering both hedonic (happiness, subjective wellbeing) and eudaemonic (positive functioning) wellbeing in general populations [37]. The items are all positively worded and have five response categories “none of the time”, “rarely”, “some of the time”,” often”, “all of the time”. The score is a sum-score with the range of 7–35 with higher scores indicating higher levels of well-being. The raw score will be converted to a metric score as recommended. The scale has shown properties of having no ceiling effect and having sensitivity to detect changes [37]. WEMWBS has been validated for use in adults from the age of 16 years in e.g. the UK [37] and Denmark [38]. It has also been found appropriate for use in adolescents from the age of 13 in England and Scotland [39]. Kuyken et al. showed effect on WEMWBS of.b in a pilot study [17]. WEMWBS is included in the latest Danish contribution [3] of “The Health Behaviour in School-aged Children (HBSC) – a World Health Organization Collaborative Cross-national Study”

Brief Resilience Scale (BRS) [40]

The BRS is a six-item measure of resilience [40]. Item responses range from 1–5. A summary score is created that averages across the six items (range = 1–5), with higher scores indicating a greater ability to bounce back when experiencing adversity [40]. The following cut-off points have been suggested: Scores from 1.00–2.99: low resilience; 3.00–4.30: normal resilience; 4.31–5.00: high resilience [40]. Windle G et al. has proposed BRS to be one of the most valid instruments to measure resilience in their review of psychometric rigor of resilience measurement scales [41]. However, the validation has only been conducted in adult populations

School connectedness and bullying items from the Danish student well-being questionnaire (DSWQ) [42]

Since 2014, The Danish Ministry of Education has monitored well-being among all Danish public-school students on a yearly basis by use of a self-developed 40-item questionnaire. From these items, Niclasen et al. has proposed a four-factor structure based on factor analysis [42]. We will use the seven-item school connectedness scale with items such as “I feel that I belong at this school” and “Most of the students in my class are kind and helpful” as outcome in our trial. Furthermore, we will use two bullying items from DSWQ as recommended by Niclasen et al. [42]

EQ-5D-Y [43]

EQ-5D is a valid instrument to measure health-related quality of life (HRQoL) for use in economic evaluations. A child-friendly generic EQ-5D-Y has been developed and has shown to be feasible in assessing HRQoL in the age of 8–15 years [43]. It comprises five dimensions: mobility (“walking about”), self-care (“looking after myself”), usual activities (“doing usual activities”), pain or discomfort (“having pain or discomfort”), and anxiety or depression (“feeling worried, sad or unhappy”). Furthermore, respondents are asked to rate their overall health on the EQ VAS, a vertical scale from 0, labelled as “The worst health you can imagine” to 100, labelled as “The best health you can imagine”. The EuroQol Research Foundation is currently working on the development of a value set in the EQ-5D-Y context, which will make cost effectiveness analysis feasible [43]

Sleep quality [44]

A scale consisting of seven close-ended questions with three ordinal response categories ranked from 1 to 3 will measure the quality of sleep. Minimum score is 7 (sleeping badly) and maximum score 21 (sleeping well) [44]

Child-Adolescent Mindfulness Measure (CAMM) [45]

The 10-item CAMM measure mindfulness. It has been developed and validated among children and adolescents 10–17 years. Lower scores indicate higher levels of mindfulness [45]

Mindfulness practice (those allocated to.b) [17]

We will use questions on adherence that have been used in former.b research [17]