Americans have always viewed a college education as an investment in a student’s future, but there’s another sort of investment going on behind-the-scenes, and it’s nearly risk free. With access to a revolving door of prospective students and a continuous supply of federal aid, some colleges are turning hopes and dreams into big returns for Goldman Sachs and other investors. Today, prospective college students — from recent high-school graduates to those returning to school after years in the workforce — often seek out opportunities that will allow them to continue working while obtaining their degree.