Table 1.

Outcomes and mediators

Variable	Question	Type	Units/categories
Primary outcome
Binge drinking episodes	Student questionnaire: How many times within the last 30 days have you been drinking 5 or more units of alcohol within one occasion?	Continuous	Episodes
Secondary outcomes
Weekly alcohol consumption	Student questionnaire: How many units of alcohol have you been drinking on each day during the last week?	Continuous	Units of alcohol
Alcohol intake at last school party	Student questionnaire: How many units of alcohol did you drink at the last high school party you attended?	Continuous	Units of alcohol
Alcohol intake at the school during last school party	Student questionnaire: How many units of alcohol did you drink at the school during the last high school party you attended?	Continuous	Units of alcohol
Proportion of first year high school students who think alcohol plays a too dominant role at the school	Student questionnaire: Do you feel that alcohol plays a too dominant role at your high school (eg, at high school parties, school bars, introduction trips, study tours, the general conversation etc.)?	Binary	Yes/no
Explorative outcomes
Intended positive side effects
Proportion of first year high school students who feel included in the social community at school	Student questionnaire: Are you part of the social community at your school?	Binary	'Yes, always' or 'yes, sometimes' vs 'occasionally', 'seldom' or 'never'
Proportion of first year high school students who feel included in the social community at school in the total student population and among students who do not drink or have a low alcohol intake (25% lowest quantile in mean weekly alcohol consumption among first year students at baseline).	Student questionnaire: Are you part of the social community at your school?	Binary	'Yes, always' or' yes, sometimes' vs 'occasionally', 'seldom' or 'never'
Unintended negative side effect
Weekly alcohol consumption	Student questionnaire: How many units of alcohol have you been drinking on each of the days during the last week?	Continuous	Units of alcohol
Consumption of drugs.	Student questionnaire: Have you ever tried to smoke marihuana, weed or pot?	Binary	Yes/no
	Student questionnaire: Have you ever tried other drugs than marihuana?	Binary	Yes/no
School party attendance	Student questionnaire: Have you ever attended a school party?	Binary	Yes/no
Mediators (determinants)
A clear alcohol policy	Manager/coordinator questionnaire: In this school year (2019/2020): Did you introduce a new or change your alcohol policy?	Binary	‘Yes, we made changes in our alcohol policy’ or ‘Yes, we introduced a new alcohol policy’ vs ‘No, we do not have an alcohol policy’ or ‘No, we have not changed our alcohol policy’
Alcohol policy communicated to students and parents	Manager/coordinator questionnaire: In this school year (2019/2020): Was the alcohol policy communicated to parents of 1st year students? Manager/coordinator questionnaire: In this school year (2019/2020): Was the alcohol policy communicated to students?	Binary Categorical	‘Yes, at a parent meeting’ or ‘Yes, written information’ vs 'No' 'Yes, all students were informed','Yes, all first year students were informed','Yes, student committees were informed', 'No'
Enforcement of the alcohol policy	Student questionnaire: Is it your experience that… Alcohol is sold at most social events at your high school? Students are denied entrance to school parties or sent home if they are visibly drunk? Nobody drinks alcohol on introduction trips? Nobody drinks alcohol on study trips? Invitations to school parties do not encourage heavy drinking?	Binary	'Highly agree' or 'agree' vs ‘neither agree nor disagree’, 'disagree' or 'highly disagree'
More alcohol-free social events, than events where alcohol is sold	Alcohol policy checklist reported by school principals Student questionnaire: Alcohol is sold at most social events outside school hours at my high school	Binary	Yes/no 'Highly agree' or 'agree' vs ‘neither agree nor disagree’, 'disagree' or 'highly disagree'
Proportion of first year high school students who overestimate the alcohol use among their peers	Student questionnaire: At your high school: How many units of alcohol do you think other young people of the same gender and school year as you drank at the last high school party you attended?	Binary	Proportion who overestimates their peers’ mean alcohol intake at the school during last school party
Proportion of first year high school students who have felt a social pressure to drink	Student questionnaire: How often have you experienced any of the situations described below? I have felt a pressure to drink more that I would like to.	Binary	'Often' or 'sometimes' vs 'seldom' or 'never'
Proportion of first year high school students who feel they can have fun without drinking	Student questionnaire: To which degree do you agree in the following…- I can have fun at a party without drinking	Binary	'Highly agree' or 'agree' vs ‘neither agree nor disagree’, 'disagree' or 'highly disagree'
Proportion of first year high school students who are familiar with the high schools’ alcohol policy	Student questionnaire: Do you know if your high school has an alcohol policy?	Binary	'Yes, we do, and I know the content' vs' yes, we do but I do not know the content', 'no, we don’t', or 'I do not know if my high school has an alcohol policy'
Proportion of first year high school students who talk to their parents about alcohol	Student questionnaire: Have you talked to your parents about your use of alcohol?	Binary	'Yes, we talk about it regularly' vs 'yes, we have talked about it once, recently', 'yes, we talked about it a long time ago' or 'no, we have never talked about it'.
Proportion of first year high school students who have agreements with their parents on how much they are allowed to drink	Student questionnaire: Do you have agreements with your parents about your alcohol consumption?	Binary	Yes/no