Every Classroom, Every Day (ECED) is a set of instructional improvement interventions designed to increase student achievement in math and English/language arts (ELA). ECED includes three primary components: (a) systematic classroom observations by school leaders, (b) intensive professional development and support for math teachers and instructional leaders to reorganize math instruction, assessment, and grading around mastery of benchmarks, and (c) a structured literacy curriculum that supplements traditional English courses, with accompanying professional development and support for teachers surrounding its use. The present study is a two-year trial, conducted by independent researchers, which employed a school-randomized design and included 20 high schools (10 treatment; 10 control) in five districts in four states. The students were ethnically diverse and most were eligible for free or reduced-price lunch. Results provided evidence that ECED improved scores on standardized tests of math achievement, but not standardized tests of ELA achievement. Findings are discussed in terms of differences between math and ELA and of implications for future large-scale school-randomized trials.