How many illegal immigrants do these countries have in their school system? How many with minorities who dont speak the main language at home? How many kids in Japan/Finland dont eat properly because their families can't afford food? Of course the American's student's results are going to be worse on average.
The fact that poor children aren't getting enough food in a affluent Western country is abhorrent, the fact the US refuses to provide comprehensive language classes to the children of immigrants is intentionally meant to disadvantage them, the fact that the US school system has a massive disparity of funding is on purpose so that those in rich neighborhoods get a better quality of education.
Of course America's students results are worse when the American people seemingly don't care about them. Maybe you should bring in a better class of politician and instead of constantly whining about your "high" taxes you pay MORE tax and help the disadvantaged. Or heck you could cancel all of your corporate welfare and ultra-rich subsidies and use that money to help the poor...
The US prides itself on being a Christian country, but they don't act like it. Christians are socialist in nature[0].
[0] http://www.faithstreet.com/onfaith/2011/08/12/from-jesus-soc...