STEM education plays a key role in influencing and orienting students' interests towards STEM fields and careers. This influence is multifaceted, involving not only STEM content and experiences but also teachers, schools, and personal factors. The purpose of this study is to explore the factors impacting the academic performance of 951 tenth grade students in STEM subjects (physics, chemistry, biology, mathematics, and information technologies) who are attending public and private schools in two provinces of Turkey. Using a correlational research model, the study assessed how students' personal characteristics, teacher effectiveness and school characteristics affect achievement in STEM courses. Data were collected using the 'Factors Affecting STEM Achievement Questionnaire (FA-STEM-A)', a 20-item questionnaire divided into three sub-dimensions. Analyses focused on the relationships between student characteristics, teaching quality, school infrastructure and STEM performance. Structural equation modelling revealed that positive student characteristics, especially fondness for STEM subjects and confidence in STEM abilities, moderately increase achievement. In contrast, negative factors attributed to teachers and schools, such as inadequate classroom activities, lack of educator expertise, inadequate technological resources, and large class sizes, hindered student performance. These findings emphasize the important influence of both inherent student qualities and external educational conditions in determining STEM education.