Purpose Although safe Internet use is an emerging public health issue, there is a scarcity of published work describing relevant school-based interventions. The objective of this study was to explore the impact of a health professional-led, school-based intervention in raising awareness on cyber-safety in adolescents, Therefore, the aim of this study was to investigate adolescents' evaluation of this school-based intervention, 6 months after its implementation, as well as the impact of adolescents' school class and gender on their evaluation. Methods A student sample was selected using a multistage stratified random sampling technique, according to the location and school grade level (middle, high school). The students - aged from 12 to 18 years old experienced an interactive presentation in their classrooms on the amount of time spent online, the use of social networks and the available support services. An evaluation tool was completed anonymously and voluntarily 6 months after the intervention. Results Four hundred and sixty-two students (response rate 90.7%, 246 middle, 216 high school) completed the evaluation tool. Younger students, especially the ones in the first year of middle school, scored significantly higher in all six parameters used in the evaluation of this intervention compared with all the older participants: (a) they had kept the presented information on Safeline and Saferinternet websites and the helpline Ypostirizo (70.2% vs. 33.7%, p < 0.001) (b) they had already used it (32.5% vs. 12.3%, p < 0.001), (c) they had learned new information on cyber safety (66.4% vs. 34%, p < 0.001), (d) they rated the intervention as more interesting (median 8 vs. 7, p < 0.05), (e) they had reconsidered the way they use Internet (median 7 vs. 6, p < 0.05) and (f) they had changed their cyber behavior (median 7 vs. 5, p < 0.05). Conclusion The active involvement of students in a discussion on cyber-safety based on their experiences was highly evaluated. The impact of the intervention on the youngest students underlines the need for raising awareness on cyber-safety and support services, earlier in the students' life.