Background: Teacher qualifications have been emphasized as a basis of professional development to improve classroom practices for at-risk children's school readiness. However, teacher qualifications have often not been compared to another form of professional development, in-service training. Objective: The current study attempts to investigate contributions of multiple types of professional development to school readiness skills of low-income preschoolers. Specifically, we examined the significance of teachers' education level, degree, teaching certificate, teaching experiences as well as specialized in-service training and coaching support as these teacher trainings are linked to preschoolers' school readiness through proximal classroom practices. Method: We used a multi-level path analysis to examine multiple pathways from teachers' professional development to classroom environments and school readiness with Head Start Family and Child Experiences Survey 2003 (N = 2,159). Results: Teachers with an early childhood education major provided higher-quality provision for learning and social-emotional practices in the classroom; teachers who received coaching provided higher-quality social-emotional and parent involvement practices. Further, children in higher-quality social-emotional classrooms had better math skills, social skills and learning behaviors; children in the classrooms with higher-quality parent involvement practices had higher receptive vocabulary and parent-reported social skills and positive approaches to learning. Conclusions: Along with early childhood education degree, ongoing coaching support would work effectively, improving classroom environments and a broad array of school readiness skills of at-risk children.