School-based mindfulness programs (SBMPs) have shown a range of academic, emotional, and behavioral outcomes. However, heterogeneity in curricular content obscures accurate interpretation of results. To unpack the "black box" of SBMPs and better contextualize intervention outcomes, this investigation examined the practices, skills, and knowledge conveyed in twelve curricula. Required curricular elements (CEs) from the written curricula were coded for mindfulness practices and skills, and lessons were coded for mindfulness knowledge conveyed. Curricula had a different composition of components depending on theoretical foundation and program length. All curricula included intrapersonal (i.e., individual) practices, most often engaging somatic awareness. Interpersonal (i.e., relational) practices were identified in eight curricula and were infrequent, except in two programs that focused on kindness/compassion. All curricula cultivated intrapersonal skills of focused attention, emotion awareness, and emotion regulation at varying levels. Programs derived from MBSR cultivated the highest proportion of skills focused on awareness of mental states. Longer programs had a significantly higher proportion of CEs focused on interpersonal skills compared to shorter programs. Longer programs also had a higher proportion of lessons concentrated on focused attention and on empathy/perspective taking. Knowledge conveyed in lessons did not always correspond to the practices and skills coded, indicating students may be learning about topics even if they are not actively practicing or engaged in skill cultivation. This study offers clarity regarding the curricular content of SBMPs, making it possible to investigate links between active ingredients and program outcomes, refine theories of change, and better prepare teachers facilitating SBMPs.