Table 1 Description of the outcome measures assessed at baseline and 3 and 6 months among students

Demographic data

Data on age, gender, school, class, and family setting (only baseline)

Warwick-Edinburgh Mental Well-Being Scale. The short version (SWEMWBS) [27, 28]

The SWEMWBS is a seven-item questionnaire measuring mental well-being [27]. All items are positively worded. Participants are asked to rate each item on a 5-point Likert scale ranging from 1 (none of the time) to 5 (all the time) when considering the last two weeks. The overall score is calculated by summing the scores for each item. A higher score indicates a higher level of mental well-being. The scale is a valid and appropriate instrument to measure mental well-being in the Danish population [27]

Strengths and Difficulties Questionnaire. The youth self-report version (SDQ) [29]

The self-report version of the SDQ is a behavioural screening questionnaire measuring subjective well-being and social, emotional, and behavioural functioning among children and youth [29]. It consists of 25 statements, some positively and others negatively worded. Participants are asked to rate each item on a 3-point Likert scale ranging from 2 (very true) to 0 (not true). The 25 items are divided into five subscales of five items each, generating scores for emotional symptoms, conduct problems, hyperactivity/inattention, peer relationship problems, and prosocial behaviour. All subscales except the last one are summed to generate a total difficulties score. The subscales range from 0 to 10 with higher values indicating poorer well-being and functioning for four of the subscales (emotional symptoms, conduct problems, hyperactivity/inattention, and peer relationship problems) and better well-being and functioning for the subscale prosocial behaviour. The total difficulties score ranges from 0 to 40 with higher values indicating poorer well-being and functioning. Goodman et al. have shown that the odds ratio for having a mental disorder was 1.23 (95% CI 1.21 to 1.25) per one-point increase in the total difficulties score and that the odds ratio for developing a mental disorder within a 3-year period was 1.16 (95% CI 1.13 to 1.18) per one-point increase in the total difficulties score [30]. In 2019, Danish SDQ norms were published [31]

Depression Anxiety Stress Scale. The short form version (DASS) [32]

The DASS short-form version is a 21-item questionnaire consisting of three subscales measuring self-rated symptoms of depression, anxiety, and stress [32]. Each subscale consists of seven items. Participants are asked to rate each item on a 4-point Likert scale ranging from 0 (did not apply to me at all) to 3 (applied to me very much or most of the time) when considering the last week. For each subscale, the points are added together and multiplied by two. Thus, it is possible to score between 0 and 42 points on each subscale [33]

Perceived Stress Scale (PSS) [34, 35]

The PSS is a 10-item self-report measure of subjective stress [34]. It consists of 10 questions indicating how often participants have found their life unpredictable, uncontrollable, and overloaded in the past month. All items are scored on a 5-point Likert scale ranging from 0 (never) to 4 (very often) calculating a total sum score between 0 and 40 with higher scores indicating higher levels of stress. The instrument has demonstrated good validity and reliability [35]

Brief Resilience Scale (BRS) [36, 37]

The BRS is a measure of resilience [36]. It consists of six statements, three positively and three negatively worded. Each item is scored on a 5-point Likert scale ranging from 1 (strongly disagree) to 5 (strongly agree). The points are added together and divided by five, calculating a total score with higher values indicating a greater ability to bounce back when experiencing adversity. The following cut-off points have been suggested. Scores from 1.00 to 2.99: low resilience. Scores from 3.00 to 4.30: normal resilience. Scores from 4.31 to 5.00: high resilience. Windle et al. have proposed BRS to be one of the most valid instruments to measure resilience [37]

Experiences Questionnaire (EQ). The decentering subscale [38]

The EQ decentering subscale is a validated 11-item self-report measure of decentering [38]. Decentering refers to the ability to observe thoughts and feelings as temporary and automatic events in the mind, rather than facts or true descriptions of reality. The items of the decentering factor assess three facets: the ability to distinguish one’s self from one’s thoughts, the ability not to automatically react to one’s negative experiences, and the capacity for self-compassion. All items are scored on a 5-point Likert scale ranging from 1 (never) to 5 (always). The total sum score ranges from 11 to 55, with higher scores indicating greater decentering [38]

EQ-5D-Y [39]

EQ-5D is a valid instrument to measure health-related quality of life (HRQoL) for use in economic evaluations. It comprises five dimensions: mobility, self-care, usual activities, pain or discomfort, and anxiety or depression. Furthermore, participants are asked to rate their overall health on the EQ VAS, a vertical scale from 0 (the worst health you can imagine) to 100 (the best health you can imagine)

Three-Item Loneliness Scale (T-ILS) [40]

The T-ILS is a short questionnaire measuring self-reported loneliness [40]. T-ILS consists of three items and is a shortened version of the UCLA Loneliness Scale consisting of 20 items [41]. Participants are asked to answer each question on a 3-point Likert scale ranging from 1 (hardly ever) to 3 (often). The points are added together calculating a total score ranging from three to nine with higher scores indicating a greater extent of loneliness [40]

Karolinska Sleep Questionnaire. Modified version [42]

Sleep problems during the past 4 weeks will be assessed with six items from a modified version of the Karolinska Sleep Questionnaire [42]. The items are responded to on a 5-point Likert scale ranging from 1 (all the time) to 5 (at no time)

Mindfulness practice (those allocated to intervention)

We will use questions on adherence which have been used in former research on mindfulness among children and adolescents [43]