While the median college graduate earns 60 to 70 percent more than the average high school graduate does, the advantage disappears for the bottom 25 percent of college-educated workers, according to a report by New York Federal Reserve Bank researchers.

"The economic benefits of a bachelor's degree still far outweigh the costs," write Jaison Abel and Richard Deitz. "However, this does not mean that college is a good investment for everyone."

Looking at wages for the lowered quartile of college graduates, "the picture is not so rosy," they note. "In fact, there is almost no difference in the wages for this percentile ranking of college graduates and the median wage for high school graduates."

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That means wages for many college graduates below the 25th percentile are less than those for a typical worker with a high school diploma, Abel and Deitz write.

"These figures suggest that perhaps a quarter of those who earn a bachelor's degree pay the costs to attend school, but reap little, if any, economic benefit."

Meanwhile, the fact that many non-wealthy students can't afford to go to college exacerbates income inequality, many economists say.

"The growth of skill differentials among the 'other 99 percent' is arguably even more consequential than the rise of the 1 percent for the welfare of most citizens," David Autor, an MIT economist, write in an article for Science magazine.

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