Recognizing the importance of in-service education and training (INSET) for teacher development, this study investigated its effectiveness in enhancing teaching efficiency among high school teachers at Southern Baptist College. Employing a convergent mixed-methods design with twenty-three participants, the study revealed that high school teachers generally perceive in-service education and training (INSET) positively and value its effectiveness in enhancing teaching skills and practices, but there is no significant relationship between the frequency of attending INSET programs and teaching efficiency. The study identified five essential guidelines that include conducting targeted needs assessments, practical application, individualized professional development plans, ongoing mentoring, and data-informed evaluation systems. The result revealed that the programs of in-service education and training attended by high school teachers were the pedagogy workshops, subject-specific training, technology integration, assessment and evaluation and professional development designed to enhance teacher’s teaching skills, subject knowledge, and classroom management techniques. Majority of the respondents attended the programs conducted by Southern Baptist College, Inc. The themes such as personal and professional growth, improved teaching effectiveness, enhanced mental abilities, positive workplace changes, transformative challenges, and emphasize collaborative approaches were drawn to address challenges and maximize benefits of ongoing professional development initiatives. Moreover, the effectiveness of in-service education and training (INSET) attended by high school teachers is perceived positively. The high school teachers in majority got Very Satisfactory in their teaching effectiveness. However, there is a weak positive correlation between the effectiveness of attending in-service training (INSET) and teaching efficiency of high school teachers. Thus, null hypothesis was not rejected.