Achievement in mathematics, particularly in advanced mathematics courses (such as trigonometry, precalculus, calculus, or Advanced Placement courses), is important for long-term college and career success. However, many students--particularly students who are Black, Hispanic, or from low-income households--do not have access to advanced mathematics courses. More broadly, college and career success depends, in part, on access to high-quality supports for postsecondary transitions, but some evidence indicates that these supports might be unequally distributed and place students who are experiencing poverty, who live in rural areas, or who have lower academic achievement levels at a disadvantage. This report presents timely and detailed information from high school principals about their students' access to and participation in mathematics instruction and postsecondary preparation supports nationally and across several states and between different school settings. The report uses nationally representative principal survey responses from the American Mathematics Educator Study, which was fielded in spring 2023 to kindergarten through grade 12 public school principals. The authors provide a national view of high school mathematics course offerings, student assignment to mathematics courses, and postsecondary transition supports, and how access to different supports for learning mathematics and preparation for college and careers varies across school characteristics and across five focal states: California, Florida, New York, Texas, and Washington. A companion report examines students' access to and participation in mathematics learning opportunities in kindergarten through grade 8.