School Digger - Great web site with a funny name. On this site you can compare Local schools by standardized test results, and a variety of other demographics. Dig to your hearts content.
http://www.schooldigger.com/go/WA/city/Seattle/search.aspx
GreatSchools.org is an excellent resource for parents of school age children. It has all kinds of articles about being a parent of a school age child.
The Office of the Superintendent of Public Instruction (OSPI) maintains a fantastic site they call the Report Card. You can look at district or school information and see a wealth of information on test scores, demographics and historical data.
http://reportcard.ospi.k12.wa.us/summary.aspx?year=2009-10
Here are web sites to specific districts:
Bainbridge Island: http://www.bainbridge.wednet.edu/
Bellevue: http://www.bsd405.org/
Edmonds: http://www.edmonds.wednet.edu/
Everett: https://www.everettsd.org/
Federal Way: http://www.fwps.org/
Highline: https://www.highlineschools.org/
Issaquah: http://www.issaquah.wednet.edu/
Kent: https://www.kent.k12.wa.us/
Lake Washington: https://www.lwsd.org/
Mercer Island: https://www.mercerislandschools.org/
Monroe: https://www.monroe.wednet.edu/
Mukilteo: http://www.mukilteo.wednet.edu/
North Shore: https://www.nsd.org/
Renton: https://www.rentonschools.us/
Seattle: https://www.seattleschools.org/
Shoreline: http://www.shorelineschools.org/
Snohomish: http://www.sno.wednet.edu/
Vashon Island: https://www.vashonsd.org/