The Expanded Susceptibility to Smoking Index (ESSI) is based on the combination of susceptibility to smoking and curiosity about smoking. The ESSI can identify young people who are at risk of starting to smoke cigarettes and related products. The objective of this study was to analyse the ESSI results and to examine factors associated with ESSI scores in students between 12 and 16 years of age. Sociodemographic, social/environmental and personal variables were analysed, and the ESSI value was determined for non-smoking students recruited from three schools in western Spain. Regression models were used to examine the factors associated with smoking for the entire sample and the factors associated with ESSI scores in the non-smoking population. Of the 377 participants who were analysed, 20.4% were smokers. Among the non-smokers, 53.5% and 55.3% presented medium-high ESSI scores for cigarettes and e-cigarettes, respectively, and 39.8% presented medium-high ESSI scores for hookah use. A higher ESSI score was associated with greater exposure to people smoking in the home, having more friends who smoke, alcohol consumption, and a higher impulsivity scale score. These findings reinforce the importance of reducing peer pressure and suggest the important role of resolve under conditions of positive affect on reducing impulsivity. Approaches based on self-efficacy could be addressed in preventive programmes developed in educational settings.