For over fifty years there have been efforts to improve mathematics achievement, those efforts focused mainly on the classroom teacher. However, growth in students’ mathematics achievement, in particular high school students’, has been unremarkable and, many believe, too slow to meet the demands of our increasingly technologically dependent society. Largely left unmentioned in efforts to improve mathematics achievement is the role that high school principals play in improving students’ mathematics achievement. This despite the fact that research has shown that principals are second only to classroom teachers in impact on student achievement. This qualitative study explores principals’ impact on high school students’ mathematics achievement. Specifically, this phenomenological study investigates how principals’ beliefs and perceptions integrate with their perceptions and beliefs about mathematics instructions to influence their support mathematics instruction. Interviews of 27 principals were transcribed and analyzed using a constant comparative method. Findings indicate that the daily demands and the increasing complexity of the principal position make engaging in activities that support improved mathematic instruction a challenge for high school principals. This study uncovered two factors complicating this difficult situation. First, principals do not rely on a theory of leadership or guiding theory of action, and second, principals struggle to name high-quality instructional strategies, suggesting that even when principals have the time to observe instruction, they may not be focusing on evidence-based practices. This is supported by the number of participating principals who believed that traditional teaching methods were acceptable and effective.