This study examined the effects of family structures on students' academic achievement in terms of self-reported grades. It also examined relevant factors that would explain the differences in student grades among students from intact two-parent families, step-families, and single-parent families. Data came from a statewide survey of students in grades 6 through 12 in Rhode Island. The predictors used were demographic characteristics (family structures, participation in free/reduced lunch program, minority status, mother's education, after-school supervision, and grade levels), parental academic expectation, family learning environment, and daily stresses. The achieved sample contained 25,511 students from 2-parent families, 4,831 students from step-families, and 8,929 students from single-parent families. Results support the hypothesis that family structure was not itself a factor in explaining the differences among the groups. Student perception of parental academic expectations was the most important predictor for differences in achievement. Students who believed their parents had high academic expectations tended to have higher grades regardless of family structure. However, a larger portion of the students from step- or single-parent families tended to have very low expectations. They also tended to experience more stresses at school, which had negative effects on achievement. The study shows that the beliefs and attitudes of parents foster the academic success of their children. (Contains 2 tables, 1 figure, and 11 references.) (SLD)