The aim of the current study was to evaluate the efficacy of the short-term school-based intervention program Try Volunteering in the development of the Five Cs (Competence, Confidence, Connection, Character, and Caring) of positive youth development. The longitudinal quasi-experimental study design was used for the pilot evaluation of the intervention program. The study sample consisted of 615 adolescents assigned to intervention and control groups. The Latent Class Growth Analysis revealed that most program participants showed an increase in Competence, Connection, and Caring as well as maintained stable levels of Confidence and Character; whereas most non-participants showed a decrease in Competence, Confidence, and Character and maintained stable levels of Connection and Caring. The effect size estimation revealed large between-group program effects on Competence and Confidence and moderate effects on Character as well as moderate within-group time effects on all Five Cs. Thus, the intervention program Try Volunteering is an effective tool for fostering positive youth development.