Ed Spending as % of GDP

NCES (National Center for Education Statistics) has some data showing how education spending has changed as a percent of GDP from 1970 to 2007.
Link: http://nces.ed.gov/edfin/tables/tab_gdp.asp

Key points:
It has been hovering in a range of 3.2% to 4.1% and was at its highest in 2007-2008. The last year covered.

There appears no trend. It was 3.9% to 4.0% in 1970's, gradually dropped to 3.2% in mid 1980's and slowly climbed back up to 4.0% by 2001 where it has pretty much stayed.