Budget reports are presented to the School Board at scheduled meetings. All reports (budget and non-budget) are housed on BoardDocs.
Click on the link and browse to "Meetings", then a given date, and then "View the Agenda" to find the monthly budget report.
The WA Office of the Superintendent of Public Instruction (OSPI) maintains historical documentation on a range of topics including enrollment, staffing, salaries, and budgets.