The curriculum of the subjects (i.e. maths, English, Norwegian) were taught in a physically active manner. The lesson should last 30 minutes and be led by the teacher of the current subject.

Without a connection to any specific subject "Physical activity" should be performed as 30 minutes a week. There are no specific aims to be taught in this lesson, but physical activities that stimulates mastery, joy and well-being should be in focus.

The Be Happy classes were self-organized activity groups of at least three students, developed according to activity preferences across regular classes. The Be Happy groups practiced traditional sports and physical activities, lifestyle sports, dancing, out-door recreation, drama etc. - inside or outside school.

Self-reported in a questionnaire. There are questions related to minutes spent in front of a screen. The answers are categorized into seven alternatives ranging from "No time at all" to "More than 6 hours per day". There are also questions about which screen activity they perform. The answers are categorized into six alternatives ranging from "No time at all" to "More than 3 hours per day". There are also questions about time spent doing homework. The answers are categorized into nine alternatives ranging from "No time at all" to " More than 7 hours per day".

Handgrip strength using a hand dynamometer. The participants will use the dominant hand, with the arm completely extended and squeeze the dynamometer with maximum isometric effort, for about 2-3 seconds.

Standing broad jump. The participants will stand behind a line with feet slightly apart. They will be instructed to perform a two-foot take-off and landing, and to jump as far as possible, landing on both feet without falling backwards. The distance from the take-off line to the nearest point of contact on the landing (back of the heels) will be measured, and the better of two attempts will be used for analyses.

Abdominal muscular endurance will be measured by a sit-up test. The participant will start in a lying position with hands clasped behind the neck, knees bent at a 45° angle with the heels and feet flat on the floor and held down by the tester. The subject then rise to a position with the elbows pointed forward until they touched the knees. The total number of correctly performed and completed sit-ups within 30 seconds is counted.

Intermittent running test. Participants run for 10 minutes, and the distance covered (in meters) is recorded.

Specific standardized Norwegian National tests in reading.The test is designed and administrated by The Norwegian Directorate for Education and Training.

Specific standardized Norwegian National tests in numeracy. The test is designed and administrated by The Norwegian Directorate for Education and Training.

Assessed with the Strength and Difficulties Questionnaire (SDQ). This is a short screening instrument consisting of 25 items equally divided across five scales measuring emotional symptoms, conduct problems, hyperactivity-inattention, peer problems, and prosocial behavior.

Each statement is answered as "Not true", "Somewhat true" and "Certainly true". Some questions have reversed form, which is reversed in the scoring of the instrument. Replies were scored from 0 to 2, and the subscales are summed to a score ranging from 0-10 for each subscale. Added together, the first four scores generate a total difficulties score, ranging from 0 to 40. Higher scores indicate more problems.

Adolescent's subjective health and well-being is assessed using the Kidscreen-27 questionnaire. The instrument consists of 27 items covering the following five quality of life dimensions: 1) physical well-being (5 items), 2) psychological well-being (7 items), 3) parents/guardians relations & autonomy (7 items), 4) social support & peers (4 items), and 5) school environment (5 items).

The questions are answered on a 5-point likert scale. Scores can be created for each of the five dimensions. The methodology given in the developers manual will be used to obtain the T-scores; mean (±SD) scores of 50 ± 10 define normality for children and adolescents aged 8-18 years across Europe. Higher scores indicate a better health-related quality of life.

Assessed through Harter's Self-perception Profile for Adolescents (SPPA). The instrument consists of seven subscales covering the following domains: 1) scholastic competence, 2) social competence, 3) athletic competence, 4) physical appearance, 5) job competence, 6) close friendship, and 7) romantic appeal.

Each subscale contains four items, each having four options: 1 (Describes me very poorly), 2 (Describes me fairly poorly), 3 (Describes me fairly well), and 4 (Describes me very well), providing a mean score of perceived competence in that area, ranging from 1 (lowest level of self-esteem) to 4 (highest level of self-esteem).

Assessed by the short version of Hopkins Symptom Checklist (HSCL-10). HSCL-10 consists of 10 symptoms or problems that people sometimes have in which adolescents answered how much the symptoms have bothered or distressed them during the last week.

All 10 questions have 4 response categories: 'Not at all', 'A little', 'Quite a bit' and 'Extremely'. The responses are summarized across all items and the mean score is used as a measure of psychological distress. Higher values indicates higher psychological distress.

The Classroom Climate Scale is used to assess learning environment. This instrument consist of 22 questions related to intrinsic and extrinsic motivation, anticipation and teacher-student relations.

All questions are answered on a four-point scale: "Strongly agree", "Somewhat agree", "Somewhat disagree", "Strongly disagree". All points are summed to a total score. Higher scores indicates higher classroom environment.

Learning environment at school in general is self-reported in a questionnaire. It consist of three separate questions, and the questions are answered on a 7-point likert scale ranging from "totally disagree" to "totally agree". A mean score from the three questions will be made, and a higher score indicates better school environment.

Measured with an ergonomic circumference measuring tape. The measure is taken at the midway between the lower rib and iliac crest with the individual's abdomen relaxed at the end of a gentle expiration.

Assessed by Basic Psychological Needs in Exercise Scale (BPNES). BPNES consist of 12 questions that are answered on a 7-point Likert scale (ranging from 1 (Certainly untrue) to 7 (Certainly true)). The responses are summarized and a mean score is made. Higher values indicates higher satisfaction.

Assessed by the Basic Emotions Trait Test (BETT). BETT consists of 9 statements that will measure positive, negative and eudaimonic emotions. Participants respond on a 7-point Likert scale ranging from 1 (Never) to 7 (Always). The responses are summarized across the three sub-scales and a mean score for each sub-scale is made. Higher values indicates higher emotions.

Assessed by Eagerness for Physical Activity Scale (EPAS). EPAS is a one-dimensional scale, consisting of 12 statements that affect emotional expressions and cognitive assessments as well as future physical activity behavior. The participants respond to the statements on a scale ranging from 1 (completely disagree) to 7 (completely agree). The responses are summarized and a mean score is made. Higher values indicates higher eagerness.