How 'bout this. Starting with 5th grade give $1000 cash per year to each student in the top 5%. Then, the best might have $8000 ready for college and stand a fighting chance of actually being able to pay for it.
They actually did that for me in high school in California. I received $1000 each year from the state for 2000 to 2002 (which was the last year of the program). It was incentive enough to care about the tests.