The percentage of adolescents aged 15 who reported being overweight or obese in Croatia, and certain Southern European countries, was around 15% in the 2009/2010 period (OECD, 2012). These results are somewhat alarming, since excessive weight can bring about serious negative health outcomes in adulthood, such as cardiovascular diseases (Baker, Olsen & Sorensen, 2007). Although sometimes overlooked because of the life-threatening aspect of physical consequences, psychological issues related to obesity should also be taken into account. For instance, being obese or overweight has been proven to have a detrimental effect on the development of self-esteem, and lead to poorer mental health later on (Wang, Wild, Kipp, Kuhle, & Veugelers, 2008).

One possible way of avoiding these negative outcomes lies in school-based prevention programs focusing on instructing children how to embrace a healthier, more active lifestyle. A bulk of previous research has discovered negative correlations between the excessive use of television and physical activity (Chen, Liou, & Wu, 2008; Koezuka, Koo, Allison, Adlaf, Dwyer, Faulkner, & Goodman, 2006). Apart from the threat of obesity and excessive weight, more time spent watching television has been linked to attentional problems (Landhuis, Poulton, Welch, & Hancox, 2007) while certain results have also shown a connection with adolescent smoking(Gidwani, Sobol, DeJong, Perrin, & Gortmaker, 2002). Similar results surfaced about computer use and playing videogames (Koezuka et al., 2006; Vandewater, Shim, & Caplovitz, 2004), though some of the studies seemed to show gender-related outcomes, in the direction of only girls' computer use being statistically significant in predicting obesity (Kautiainen, Koivusilta, Lintonen, Virtanen, & Rimpela, 2005).

Since children spend a substantial amount of time at school at this age, it seems as a good environment to educate them on the risks pertaining to the excessive use of television and computers in their free time, as well as promoting the benefits of a more physically active lifestyle. Several papers have addressed this by evaluating health-promoting interventions in school-aged children.

Our study sought to investigate whether a more general educational program, which addressed the benefits of physical activities and promoted a healthy lifestyle among other issues, would bring about the desired changes in students' habits. Firstly, we evaluated the relationship between physical exercise and mediause in a sample of older students and then used a quasi-experimental design to ascertain whether the students included in the experimental condition changed their habits regarding time spent on exercise and screen-based activities after the intervention, as opposed to the control group who did not receive any instructions on the matter.

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