Jason L. Riley writes: Though schools ought to be more discriminating about whom they admit, student financial-assistance programs push them to admit students who are not prepared to succeed. In 1970, about 12% of recent college grads came from the bottom 25% of the income distribution. Today, it’s about 10%. “We’ve had a decline in poor people graduating from college. More poor people are attending, but fewer are graduating. We have not really improved making college a vehicle for achieving the American dream.”