This study aimed to evaluate the effectiveness of a student leadership training program for the early adolescent students in school. A quasi-experiment design with a pre-test and post-test control group and experimental group was adopted to demonstrate the effectiveness of the program. The sample group consisted of 50 adolescent students who were 10 to 14 years old, were studying in Primary 4, 5, or 6 in formal schools, and were studying at English Tutorial School in Kanchanaburi Province, Thailand. They were assigned to 2 groups with 25 students in an experimental group and 25 students in a control group by simple random sampling method. The results showed that the intervention student leadership training program, which consisted of an online course and an on-site course with 12 activities such as "me in the photo", "rock paper scissors posse," and "share the vision", for 24 hours over 7-weeks, resulted in a statistically significant (p < .001) increase in student leadership skill. Students involved in the student leadership training program were impacted positively in terms of their leadership capabilities. The recommendations from the findings can help educational institutions, administrators and teachers to enhance students' leadership by including core activities on developing student leadership in the school curriculum.