One potentially underestimated aspect of resource inequity in U.S. public schools is access to social capital in external organizational environments. This research examines partnerships among 211 New York City high schools and 918 partner organizations from 2001 to 2005 as sources of external school social capital providing resources that can strengthen organizational capacity to improve educational opportunities and outcomes. The findings, based on an innovative analysis combining content analysis, social network analysis, and multilevel modeling, demonstrate that four partnership characteristics are important in this context: (1) how long partnerships last versus how many there are, (2) partners concentrating resources in a particular area versus across diverse complementary areas, (3) partners being densely connected to other schools and partners rather than being central in the overall school-partner network, and (4) partners conveying instructional resources versus other kinds of resources. Hence, educational research and policy should more broadly conceptualize how schools' external organizational environments matter for educational equity and the role particular kinds of partnerships can play.