This study assessed the extent to which support exerts direct or indirect effects on child and adolescent achievement (grade point average and Statistical Aptitude Test scores). Personal interviews were conducted with 120 African American, 101 Anglo-European American, and 112 Latin American students (151 males and 182 females) in grades 1-2, grades 4-5, and grades 8-9 from a multiethnic public school population. Interviews included measures of social support, life stress, loneliness, and self-concept. Forty-nine students were reinterviewed after 1 to 2 weeks to establish the social support measures' test-retest reliability. Support was related to achievement directly and indirectly in relation to loneliness and self-concept, but specific effects varied by age and ethnicity. Support effects were stronger with age and stress effects declined with age, suggesting increased ability to use support resources in coping with stress. Some ethnic group variation may be related to the significance of differential sources of support. Further research is needed to validate these results and explore proximal determinants of ethnic group differences. Included are 19 references, 5 tables, and 1 figure. (RLC)