The current experience sampling study examined the change in distinct daily positive and negative emotions of 136 students (aged 10-12) participating in the Flourishing Students program. Students completed smartphone-based questionnaires covering emotions, daily activities, and social context at eight semi-random intervals for six days. Using a cluster randomized design, 70 students participated in 32 well-being lessons, and 66 students followed their standard curriculum. The findings suggest that the intervention had a buffering effect on the increase in daily negative emotions that is typical during early adolescence. Moreover, participants in the program experienced positive changes in feeling less lonely and increased levels of calmness and enjoyment of being alone. Noticed in several daily activities were similar differences between the groups. The intervention effect was slightly more pronounced in girls than boys. Overall, we hope to encourage incorporation of positive psychology into education and to broaden the scope of evaluation methods.