The returns to education are very high (Grossman, Becker, Murphy estimate an 18% return to an additional year of school for the marginal american highschool student (marginal here meaning one who has the lowest return of those staying in school---or the highest of those dropping out)). Even poor parents are better off investing in their children than in a savings account--assuming that they believe their children will 'repay' that debt. But they often can't borrow money to invest in their kids.
Why don't rich people/hedge funds/etc invest in the education of poor kids?
I am glad to see that Monetizing Emma has taken on the question.