As a percentage of Gross Domestic Product, the United States is outspending all other countries. So, it can't be just about needing more money for education.
http://www.nationmaster.com/graph-T/edu_tot_exp_as_of_gdp&int=-1
Education: Total expenditure as % of GDP
1. United States 7.0% of GDP
2. Denmark 6.7% of GDP
3. Sweden 6.5% of GDP
4. Canada 6.4% of GDP
5. France 6.1% of GDP
6. Australia 6.0% of GDP
7. Norway 5.9% of GDP
8. New Zealand 5.8% of GDP
9. Switzerland 5.7% of GDP
10. Austria 5.7% of GDP
11. Finland 5.6% of GDP
12. Belgium 5.5% of GDP
13. Germany 5.3% of GDP
14. United Kingdom 5.3% of GDP
15. Italy 4.9% of GDP
16. Netherlands 4.7% of GDP
17. Japan 4.6% of GDP
18. Ireland 4.6% of GDP
It's also interesting to note that none of the countries in the above list are even close the United States in population! Many of the countries listed have less of a population than some of our largest states.
http://www.nationmaster.com/graph-T/peo_pop&int=-1
(population rankings chart)