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.
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.
Here are web sites to specific districts:
Bainbridge Island: http://www.bainbridge.wednet.edu/
Federal Way: http://www.fwps.org/
Lake Washington: https://www.lwsd.org/
Mercer Island: https://www.mercerislandschools.org/
North Shore: https://www.nsd.org/
Vashon Island: https://www.vashonsd.org/