1. A school system wants to implement a policy that includes rewards and sanctions for teachers' contributions to students' achievement. The school system approached you saying they heard a value-added framework could be useful for this type of policy. What are some challenges of using value-added models to isolate teachers' unique contributions to student learning? [Note: you don't have to include a laundry list of all possible complications. But in a short paragraph, what are some key things the school will need to consider].