This paper studies the early implementation of a school improvement effort in two high schools. We examine what explains variation in the teacher adoption of program practices. Our findings highlight the tension between encouraging immediate adoption of program practices and the longer term goals of schoolwide culture change. We find that highly structured practices and those that are already aligned with teachers' extant beliefs and classroom practices can be implemented with little preexisting capacity. These conditions could also lead to more consistent and quicker initial adoption. However, this type of implementation might not encourage sufficient understanding of program goals and may inhibit the diffusion of practices into the school culture. Findings highlight dilemmas associated with program practices when the goal is to bring educational reforms to scale.