The main objective of this research is to understand the development of the critical thinking skills and self-efficacy of eleventh grade students at a girls’ school in Bangkok by using classrooms implemented with active learning pedagogy through gamification on the topic of elementary counting principles. The study uses a mixed method pre-experiment research design, which also acquires in-depth data from interviews. The study involved 29 out of 182 students retrieved from cluster sampling. The data was analyzed using descriptive statistics, including mean, percentage, and standard deviation. A t-test was chosen as the inferential statistic for data analysis. The results indicated that students’ critical thinking skills were improved (Sig. (1-tailed) 0.011 < 0.025), while self-efficacy remained unchanged (Sig. (2-tailed) 0.148 > 0.050). The in-depth data from the interviews suggests that the students’ self-efficacy did not develop because of the stress from their school life. As the findings revealed the effectiveness of active learning pedagogy with gamification toward critical thinking and self-efficacy, the use of this combined approach in schools and universities is recommended to promote the competencies needed in the 21st century. We also suggest further management of stressful academic environments, which will benefit both students and teachers. Furthermore, to obtain more practical data, a more specific context of self-efficacy and critical thinking skills could be used.