This study aimed to identify the classroom and school characteristics that are associated with student achievement in mathematics and Serbian language in primary schools in Serbia. The study sample consisted of 119 public primary schools, 253 classrooms, and 4,857 third-grade students. Variables from 3 past research paradigms--input-output, effective schools, and instructional effectiveness--as well as key student background variables, were first organised in a conceptually integrated model of school effectiveness. Then, they were simultaneously examined as predictors of student achievement in a 3-level Hierarchical Linear Model (HLM). This research found little association between school-level variables and student achievement. Several classroom-level policy-malleable variables were found to have small positive associations with student achievement: clear and structured classroom instruction, emphasis on complex (as opposed to basic) skills, whole-class instruction (rather than individual or group work), teacher feedback, orderly climate, moderately frequent reinforcement of student effort, and--to a lesser extent--use of a variety of teaching tools. (Contains 2 figures, 6 tables and 1 note.)