Effective professional development has been shown to improve instruction and increase student academic achievement. The Train the Trainer professional development model is often chosen by the state Department of Education for its efficiency and cost effectiveness of delivering training to schools and districts widely distributed throughout the state. This is a study of the Train the Trainer component of an innovative K12 professional development model designed to meet the needs of the state's lowest performing schools that served some of the state's most marginalized students. Pursuing a Vygotzkian social constructivist framework, the model was developed and informed by its stakeholders, providing training that was collaborative, job-embedded, ongoing, and continuously adapted to meet the needs of the School Improvement Grant participants. Schools in the multi-case study were awarded the federal ARRA School Improvement Grant in 2010. Focus questions include: What influence does the Train the Trainer component have on classroom instruction specifically as it relates to formative assessment? and To what extent does the trainer support the implementation of the Train the Trainer professional development at the classroom level? The action research study took place from August 2011 to February 2012 and used a mixed-methods research design.