Miami-Dade Public Schools squandered tens of millions of dollars on a mangled construction program, delayed crucial projects by months or even years, and trapped children in schools that are not only crowded, but obsolete, poorly maintained and in some cases downright unsafe, a Herald investigation has found. Questionable policies, practices and politics have repeatedly hobbled construction in the nation's fourth-largest school system, even as student enrollment soared and the need for new or renovated classrooms bred near-emergencies on campuses across the county.