A generally agreed-upon tenant of the physics teaching community is the centrality of subject-specific expertise in effective teaching. However, studies which assess the content knowledge of incoming K-12 physics teachers in the U.S. have not yet been reported. Similarly lacking are studies on if or how the demographic makeup of aspiring physics educators is different from previously reported analyses of the actual high school physics teaching workforce. Here we present findings about the demographics and subject knowledge of prospective high school physics teachers using data from Praxis physics subject assessments administered between 2006 and 2016. Our analysis reveals significant variations in exam participation and performance between men and women, as well as those with different undergraduate majors and academic performance over the past decade. Findings from this work inform understandings and decisions about the quality, recruitment, and preparation of the high school physics teaching workforce.