Table 1 Overview of questionnaire items and response categories

 Sitting time at school

On average, how long do you spend sitting during classes at school per day (do not include study time at home)?

Single item, recoded into a quantitative variable (min/day) using the midpoint method (e.g., ‘1–2 h’ = 90 min)

1 = None, 2 = 1–15 min/day, 3 = 15–30 min/day, 4 = 30–60 min/day, 5 = 1–2 h/day, 6 = 2–3 h/day, 7 = 3–4 h/day, 8 = 4–5 h/day, 9 = 5–6 h/day, 10 = 6-7 h/day, 11 = More than 7 h/day (recoded into 420 min)

 Number of breaks in sitting time at school

During a normal school lesson, how often do you usually stand up. stretch or walk around a bit?

Single item, recoded into a quantitative variable (times/h)

1 = Never, 2 = Once, 3 = twice, 4 = three times, 5 = 4 times or more (recoded into 4)

 Screen-time

Roughly how many hours a day do you usually spend watching TV/DVD in your leisure time? (Weekdays and weekend days separately)

Roughly how many hours a day do you usually use a computer/games console for leisure activities? (Weekdays and weekend days separately)

Separate items were recoded into quantitative variables (min/day) and following formula was used to assess screen-time: ((TV time + computer time on a weekday)*5) + ((TV time + computer time on a weekend day)*2), divided by 7.

1 = None, 2 = Less than 30 min/day (recoded into 15 min), 3 = 30 min/day, 4 = 1 h/day, 5 = 1 h 30 min/day, 6 = 2 h/day, 7 = 2 h 30 min/day, 8 = 3 h/day, 9 = 3 h 30 min/day, 10 = 4 h or more per day (recoded into 4 h)

 Preference to break up sitting time

I like breaking up the time I spend sitting.

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Attitude towards breaking up sitting time

Breaking up the time I spend sitting is good for me.

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Self-efficacy to break up sitting time

I find it hard to break up the time I spend sitting.

Single item

1 = Strongly agree, 2 = Partly agree, 3 = Neither agree nor disagree, 4 = Partly disagree, 5 = Strongly disagree

 Habit of breaking up sitting time

Breaking up the time I spend sitting is something that I do without even really thinking about it.

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Relationship with classmates

My classmates like being together

Most of my classmates are friendly and helpful

My classmates accept me as I am

Mean value of three items (Cronbach’s alpha at pre-, mid-, post-test = 0.68, 0.66 and 0.78)

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Involvement in organising school activities

The pupils in our school are involved in organising school activities

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Relationship with teachers

My classmates and me have good bonds with the teachers at school

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Frequency per week

How many times per week do you stand at the standing desk (From Monday to Friday)?

Single item, recoded into a quantitative variable (times/week)

1 = Never (recoded into 0), 2 = Almost never (recoded into 0), 3 = 1 time a week, 4 = 2 times a week, 5 = 3 times a week, 6 = 4 times a week, 7 = once every day (recoded into 5), 8 = more than once every day (recoded into 10)

 Duration a time

If you stand at the standing desk, how long a time do you stand up?

Single item, recoded into a quantitative variable (min/a time)

1 = 1–15 min a time (recoded into 8 min), 2 = 16–30 min a time or half a lesson hour (recoded into 25 min), 3 = 31–45 min a time (recoded into 38 min), 4 = 46–60 min a time or an entire lesson hour (recoded into 50 min), 5 = more than 1 h a time (recoded into 60 min)

 Preference to use the standing desk

I like taking lessons while standing at the standing desk

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Self-efficacy to use the standing desk

I find it difficult to take lessons while standing at the standing desk

Single item

1 = Strongly agree, 2 = Partly agree, 3 = Neither agree nor disagree, 4 = Partly disagree, 5 = Strongly disagree

 Attitude towards using the standing desk

Taking lessons while standing at the standing desk is good for me

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Habit of using the standing desk

Taking lessons while standing at the standing desk is something that I do without thinking about it.

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree

 Subjective norm related to using the standing desk

My classmates think it is good that I take lessons while standing at the standing desk

Single item

1 = Strongly disagree, 2 = Partly disagree, 3 = Neither agree nor disagree, 4 = Partly agree, 5 = Strongly agree