Two D's and you're out. At least five Palm Beach County principals' jobs are on the line this year as the school year moves into its home stretch, Superintendent Art Johnson said. Two years, he believes, is plenty of time for middle- and high-school principals to turn their schools around, even if they have long-standing reputations of student underachievement. "My position has been double-D schools get a new administrator," Johnson said. "Sometimes my staff says two years is not enough. At other schools, a principal has made a change in almost no time." Johnson's prime example is Jon Prince, who...