Emotional support from classmates and teachers is a powerful protective factor in averting or reducing student mental health problems. Yet, longitudinal evidence indicates that there is decreased support from these groups as students advance to higher grade levels, a change that may be linked to diminishing mental health. This study followed 2,616 students from 23 high schools to test the hypothesis that perceptions of declining classmate and teacher support are associated with declining mental health. Growth curve analysis revealed significant decreases in support and self-esteem and increases in symptoms of depression and social anxiety. Boys demonstrated steeper declines than girls in classmate support and self-esteem and sharper increases in depression. As hypothesized, declining classmate and teacher support was associated with worsening self-esteem and depression. Only declines in classmate support were associated with increases in social anxiety. Results were similar across gender categories. Implications for school-based practices targeting social support are discussed. © 2011 Wiley Periodicals, Inc.