Sandy Baum writes: The realities of student debt in our country make it clear that proposals to eliminate these obligations do not meet the criteria for progressive policies. Households in the upper half of the income distribution hold more student debt than those in the lower half. The highest-income quartile of households owes about one-third of that debt; the lowest-income quartile owes about 12 percent. People who don’t go to college don’t have student debt. They have lower incomes and more constrained job opportunities than others.