The constitution grants the federal government power to pass laws such as educational standards for the entire country. There is also the problem of a state's curriculum not being up to par where the nation needs to be in order to have an educated populous. The constitution is designed to promote the nation's wellbeing, and therefore any law doing that can be justified as consitutional.
I feel that I learn best when I can get into the subject by more then just tests and reading essays. A free curriculum gives students the freedom to learn the best way they can, and it gives them the opportunity to put their own beliefs and opinions into school.
I do not think that US education should be standardized. I think that a national standardized curriculum are not constitutional. The 10th amendment makes it clear that public education is something that is left for the state department to fund and make decisions. Each state or local entity should be responsible for education.