To advance the research examining the sluggish cognitive tempo (SCT) construct, a key priority has been to develop assessment tools that are reliable and valid. The current study builds upon existing work by conducting the most thorough psychometric evaluation to date of the teacher-reported Child and Adolescent Behavior Inventory (CABI) SCT and attention-deficit/hyperactivity disorder inattention (ADHD-IN) modules in a large sample of elementary students. Participants were 7,613 students (grades 2-5; 50.3% boys) attending 24 elementary schools in three school districts. Teachers (N = 398) provided ratings of SCT, ADHD-IN, academic impairment, and social impairment. An a priori two-factor model with cross-loadings found the SCT items to demonstrate excellent structural validity with ADHD-IN items. The measurement properties of the SCT and ADHD-IN constructs were also invariant across sex and grade. SCT and ADHD-IN were both uniquely associated with academic and social impairment. Graded response item response theory analysis indicated that the SCT and ADHD-IN scales provided a high level of information and precision. The current study replicates and extends previous research and provides the strongest psychometric evidence to date of teacher-rated SCT using the CABI. The teacher-report CABI may be especially useful in the school-based screening of SCT and ADHD-IN. [This paper was published in "Psychological Assessment" v32 p1047-1056 2020.]