Purloined from a listserv:
NCES has a database of school district data. Additional profiles/geographic info available here.
School District Data compiles data from the Council of Chief School Officers. Free, and much of the info is the same as the NCES.
Estimates used for funding allocations for school districts based on the ACS are available through the Small Area Income and Poverty Estimates (SAIPE).
Proximity One has some free data on the site, and sells software for geographic applications by subscription. Has 2008 school district population estimates from the SAIPE in a ranked table.