This article examines the process of shifting to a "Physics First" sequence in science course offerings in three school districts in the United States. This curricular sequence reverses the more common U.S. high school sequence of biology/chemistry/physics, and has gained substantial support in the physics education community over the past few decades. Using qualitative case study methodology, the present study focuses on the lessons learned in three school districts that successfully rearranged their course offerings and made physics a ninth-grade subject for all of its students. Findings show that in all districts, the shift was undertaken to support student learning in mathematics and in future science learning. In every case, the coordination between ninth grade physics and ninth-grade algebra was much more difficult than expected. Also, during most transitions, the number of students taking biology dropped precipitously for a period of 1--2 years. Though there is shared agreement about Physics First as the realignment of the high school curricular sequence, there is less consensus about how such programs ought to be aligned with mathematics curricula. The article concludes with suggestions for sources of evidence in conducting effectiveness studies on the Physics First approach.