If you have low quality students, who are only expected to score at the 25% level, and you boost them up to 35%, you win. If you have high quality students expected to reach 75%, but the only achieve 65%, you lose. Do you believe it's really that hard to predict that Asian kids in 2 parent $250k/year+ households will score higher than black kids in 1 grandparent $10-20k/year households?
If you truly believe the effects of education are immeasurable, perhaps we should stop wasting money on it. We don't spend $859 billion on homeopathy for much the same reason.
Developer productivity is really hard to measure with universal statistics -- every attempt to do so has failed miserably and usually made things worse. Does that mean that nobody should spend money on development?
Incidentally, measuring developers is done routinely. Every bank (except maybe Citi) has a set of procedures and guidelines for outsourced development. These procedures make the process fairly predictable and they reduce costs/dependencies.
Much like outsourced IT, teaching is repetitive grunt work. It can and should be defined and measured.
How could that ever go wrong :)
EDIT: More seriously, if it's already impossible to measure, I fail to see how more complicated metrics would do anything but do an even worse job of measuring. There might be exceptions to this rule in a few cases but it's generally the way things work.