How can student housing be going up-market at the exact moment when we are having a national freak-out over rising college costs and the staggering amounts of student debt? Such financial concerns don’t seem to have deterred schools from peppering their campuses with lavish new dorms. Just the opposite. Building student housing has become a big business, and most of the new construction is being done by private developers. Those companies have been responsible for 20 percent of new college housing since 2012, a recent survey of 40 schools found.