…. This is Rocketship Discovery Prep, one of five charter elementary schools founded by Danner that are bridging the achievement gap — the staggering difference in academic performance between poor and privileged children. …But some wonder if five-year-old Rocketship is producing miracles or mirages. Will a model that succeeds in San Jose also flourish in Nashville? Can a strategy that works for a handful of schools be expanded across the country? And can the achievement gap be eliminated? …. For two hours each day, students are taught by computers designed to meet children at their particular level and drill them in rote skills like addition or subtraction. … Computers shave 25 percent from Rocketship’s labor costs — savings used to extend the school day to eight hours, pay higher salaries to its nonunion teachers and to construct its own school facilities, among other things. … Stephen McMahon, president of the San Jose teachers union, worries that a dual system is developing: One filled with charters that attract motivated families and another of traditional public schools populated with reluctant learners….