Objectively measuring learning and "teaching to the test"

[The Tradition]

There has been an endless amount of teeth-gnashing and howls of indignation regarding standardized testing as a way to measure achievement. Teachers complain that they're forced to "teach to the test" instead of teaching the material.

I never really understood this complaint... if the material students are supposed to learn is on the test, then "teaching to the test" will result in students learning the material, right? But I do understand that perhaps the testing method/timing may not really tell us anything.

So what about this: Students take a test based on the course curriculum on the first day of class, and then they take the same test again on the last day of class.

Compare the before and after scores. This conclusively tells you if the student increased his or her knowledge by sitting in the teacher's class all year, and teacher effectiveness can be objectively measured against peers teaching the same subject.

What could possibly be wrong with testing in this manner? Wouldn't that be the most fair and objective way to measure if our kids are learning the material or not, and if our teachers are effective educators or not?