One fast-growing American industry has become a conspicuous beneficiary of the recession: for-profit colleges and trade schools . . . But the profits have come at substantial taxpayer expense while often delivering dubious benefits to students, according to academics and advocates for greater oversight of financial aid. Critics say many schools exaggerate the value of their degree programs, selling young people on dreams of middle-class wages while setting them up for default on untenable debts, low-wage work and a struggle to avoid poverty. And the schools are harvesting growing federal student aid dollars, including Pell grants awarded to low-income students.