Emotional and behavioural problems were assessed using the Strengths and Difficulties Questionnaire (SDQ), a 25-item self-report instrument comprising five subscales: Emotional Symptoms, Conduct Problems, Hyperactivity/Inattention, Peer Problems, and Prosocial Behaviour. An example item is "I worry a lot." Items are rated on a 3-point Likert scale (0 = Not true, 1 = Somewhat true, 2 = Certainly true). Subscale scores are calculated by summing relevant items (range 0-10 per subscale), and a Total Difficulties score (range 0-40) is derived by summing the four problem subscales (excluding Prosocial Behaviour), with higher scores indicating greater emotional and behavioural difficulties.

Executive function was assessed using the Amsterdam Executive Function Inventory (AEFI), a 13-item self-report questionnaire designed to measure executive functioning in adolescents. The AEFI comprises three subscales: Attention, Self-Control, and Planning. An example item is "I have difficulty concentrating for a long time." Items are rated on a 3-point Likert scale (1 = Not true, 2 = Partly true, 3 = True). Subscale scores are calculated by summing relevant items, and a total executive function score can be derived by summing all items, with higher scores indicating better executive functioning (after reverse scoring negatively worded items as appropriate).

Affective attitude toward interrupting sedentary behaviour during school breaks was assessed using three bipolar adjective pairs (unpleasant-pleasant, boring-interesting, and unenjoyable-enjoyable) adapted from established Theory of Planned Behaviour questionnaires. Participants responded to a common stem ("In the next week, interrupting sitting by standing up and moving during routine class breaks is …"), followed by three items. Responses were recorded on a 7-point Likert scale (1 = strongly disagree, 7 = strongly agree). A mean score was calculated across the three items, with higher scores indicating a more positive affective attitude toward taking active breaks.

Instrumental attitude toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using three bipolar adjective pairs (harmful-beneficial, worthless-valuable, and bad-good) adapted from established Theory of Planned Behaviour measures. Participants responded to a common stem ("In the next week, interrupting sitting by standing up and moving during routine class breaks is …"), followed by three items (e.g., "In the next week, interrupting sitting by standing up and moving during routine class breaks is harmful-beneficial"). Responses were recorded on a 7-point Likert scale (1 = strongly disagree, 7 = strongly agree). A mean score was calculated across the three items, with higher scores indicating a more positive instrumental attitude toward taking active breaks.

Perceived capability toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using three items adapted from established Theory of Planned Behaviour measures. Participants responded to a common stem ("In the next week, interrupting sitting by standing up and moving during routine class breaks is …"), followed by items assessing perceived ease and capability (e.g., "It is easy for me to interrupt sitting by standing up and moving during routine class breaks in the next week"). Responses were recorded on a 7-point Likert scale (1 = strongly disagree, 7 = strongly agree). A mean score was calculated across the three items, with higher scores indicating greater perceived capability to take active breaks.

Intention toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using three items adapted from established Theory of Planned Behaviour measures. Participants responded to a common stem ("In the next week, interrupting sitting by standing up and moving during routine class breaks is …"), followed by items assessing behavioural intention (e.g., "I intend to interrupt sitting by standing up and moving during routine class breaks in the next week"). Responses were recorded on a 7-point Likert scale (1 = strongly disagree, 7 = strongly agree). A mean score was calculated across the three items, with higher scores indicating stronger intention to take active breaks.

Perceived opportunity toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using three items adapted from a Multi-Process Action Control (M-PAC) questionnaire applied to physical activity. Items captured students' perceived availability of opportunities to interrupt sitting during routine class breaks over the subsequent week (e.g., "If I really wanted to interrupt sitting by standing up and moving during routine class breaks over the next week, I would have the opportunity to do so"). Responses were recorded on a 5-point Likert scale (1 = strongly disagree, 5 = strongly agree). A mean score was calculated across the three items, with higher scores indicating greater perceived opportunity to take active breaks during school breaks.

Planning toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using six items adapted from questionnaires based on the Health Action Process Approach. Items assessed students' use of planning strategies related to interrupting sitting during routine class breaks over the previous week (e.g., "I made regular plans concerning how to interrupt sitting by standing up and moving during routine class breaks in the last week"). Responses were recorded on a 5-point Likert scale (1 = strongly disagree, 5 = strongly agree). A mean score was calculated across the six items, with higher scores indicating greater use of planning strategies for taking active breaks.

Self-monitoring toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using three items adapted from questionnaires based on the Health Action Process Approach. Items assessed students' engagement in monitoring their break-time sedentary interruption over the previous week (e.g., "I kept track of whether I interrupted sitting by standing up and moving during routine class breaks over the last week"). Responses were recorded on a 5-point Likert scale (1 = strongly disagree, 5 = strongly agree). A mean score was calculated across the three items, with higher scores indicating greater use of self-monitoring strategies for taking active breaks.

Identity toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using four items adapted from the Exercise Identity Scale. Items captured the extent to which engaging in break-time sedentary interruption was integrated into students' self-concept (e.g., "When I describe myself to others, I usually include that I interrupt sitting by standing up and moving during school breaks"). Responses were recorded on a 5-point Likert scale (1 = strongly disagree, 5 = strongly agree). A mean score was calculated across the four items, with higher scores indicating stronger identity integration related to taking active breaks.

Habit strength toward interrupting sedentary behaviour during school breaks (i.e., taking active breaks) was assessed using twelve items adapted from the Self-Report Habit Index. Participants responded to a common stem ("In the last month, interrupting sitting by standing up and moving during routine class breaks was …"), followed by items assessing behavioural automaticity and repetition (e.g., "…something I do frequently"). Responses were recorded on a 5-point Likert scale (1 = strongly disagree, 5 = strongly agree). A mean score was calculated across the twelve items, with higher scores indicating stronger habit strength for taking active breaks during school breaks.