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.

http://www.greatschools.org

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/