The System for Observing Fitness Instruction Time (SOFIT) is commonly used to measure variables related to physical activity during physical education (PE). However, SOFIT does not yield detailed information about teacher practices related to children's moderate-to-vigorous physical activity (MVPA). This study describes the modification of SOFIT (referred to as SOFIT+) to measure teacher practices related to students' MVPA during PE lessons. Teacher practices that (a) promote and (b) limit children's MVPA were identified and operationally defined via a literature review and Delphi Method. Reliability and validity data for SOFIT+ were obtained from 20 video recorded elementary PE lessons. Validity of teacher behaviors included in SOFIT+ was established using students' accelerometer-derived MVPA. Twenty-seven teacher practices were identified. Inter-rater agreement ranged from 75.8% to 99.1% across 724 reliability scans. Six and seven practices were independently related to girls' and boys' MVPA, respectively. For example, during elimination activities, boys and girls were less likely to engage in MVPA. Boys were up to 1.51 times more likely to engage in MVPA while girls were up to 2.86 times more likely to engage in MVPA when a greater number of teacher practices were observed concurrently. Findings provide preliminary reliability and validity for SOFIT+, an instrument to comprehensively measure teacher practices related to children's MVPA during PE.