The increasing trend to mainstream students with educational challenges (SEC) has obligated mainstream teachers to develop different approaches to deal with heterogeneous classes. Previous research on in-service and pre-service teachers has shown that successful inclusion is correlated with positive attitudes towards inclusion, a high sense of self-efficacy, and minimal stigmatization of SEC. However, it remains unclear whether training programs for in-service and pre-service teachers can impact initial attitudes towards inclusive education. This study examined the different components of attitudes towards inclusion in three groups: in-service teachers, pre-service teachers, and college students enrolled in non-education majors. An online questionnaire was administered by a survey company to 489 participants: in-service teachers who had at least one SEC in their classes (n=196), pre-service teachers (n=143), and college students (n=150). The results showed that pre-service teachers had the most positive attitude towards inclusion of SEC, whereas in-service teachers had the most negative attitude. Attitudes towards inclusion were predicted by stigmatizing attitudes and self-efficacy. Stigmatizing attitudes were predicted by gender, religion, education, and self-efficacy. Thus, the main route to more inclusive and less prejudiced teachers may lie in strengthening teachers' self-efficacy. These results have implications for training programs, which should focus on eliminating myths, strengthening the concept of inclusion throughout the entire program, and enhancing the expertise of in-service teachers.