Meta-analyses and reviews have explored school-based mindfulness program (SBMP) outcomes, but the contents of the written curricula remain unexamined. The purpose of this study was to identify and categorize the mindfulness practices and skills as evidenced in the written curricula of twelve accessible SBMPs. Three coders divided each curriculum into curricular elements (CEs) and coded each element for instructional mode and, where applicable, mindfulness practice and skill. Descriptive analyses were conducted on codes to ascertain the frequency of mindful skills and practices across all SBMPs and differences between elementary and secondary school curricula. Of the 2643 CEs identified across curricula, 50.1% were coded as a mindfulness practice. Twenty distinct mindfulness practices were coded within six practice-type categories. The Somatic category included the most distinct practices and represented the majority (67.6%) of mindfulness practices taught. Over 73% of CEs were coded as fostering a mindfulness skill. Intrapersonal skills were most often cultivated across SBMPs with "focused attention" being most prevalent. Results suggest there may be a reliance on more straightforward practices and skills that do not require the teachers' or students' grasp of the complex nuances of mindfulness. Developmental differences show more prevalence of interpersonal skills for elementary students than secondary. This analysis of core curricular content of SBMPs provides foundational knowledge necessary for refining research at all stages of the NIH Stage model, from theory generation and refinement based on children's developmental needs to identification of active ingredients most likely to impact intervention outcomes.