Introduction A low proportion of bystanders in Slovenia are willing to provide resuscitation to people experiencing cardiac arrest. We measured knowledge acquired after cardiopulmonary resuscitation training among Slovenian children in the final three grades of primary school. Methods This pre- and post-test cohort study included 566 schoolchildren aged 12–15 years. From April to May 2018, we administered a 15-item questionnaire to children in 15 primary schools, to assess the effects of theoretical and practical cardiopulmonary resuscitation training on their knowledge of cardiopulmonary resuscitation at 1–2 months after training. Data were processed using univariate, bivariate, and multivariate analyses. Results A significantly greater level of post-training knowledge was noted in all three equally sized school grade groups (p=0.001). The youngest group (mean age 12.5 years) exhibited the greatest increase in knowledge, with test scores increasing by an average 2.65 (range 0–15) points. Age (p=0.001), body mass index (p=0.037), female sex (p=0.006), and previous resuscitation training (p=0.024) were significant independent predictors of pre-training knowledge level. Sex was the only predictor significantly influencing knowledge levels after training (p=0.002); girls scored up to 0.7 points higher than boys, both before and after training. Conclusions Among Slovenian schoolchildren aged 12–15 years, a significantly improved level of theoretical knowledge was demonstrated after cardiopulmonary resuscitation training. The introduction of cardiopulmonary training may be most effective in children aged 12.5 years (seventh graders). Early, compulsory resuscitation training might reduce social barriers to performing resuscitation, which may eventually translate into better post-cardiac arrest outcomes.