The San Diego Unified School District is planning to flush pipes with water at schools prior to the City of San Diego administering tests for lead. District officials say the practice will protect students, while water quality experts warn it could hide dangerous levels of the metal. The San Diego Unified School District began testing its schools’ water Tuesday after lab reports confirmed “higher than allowable” lead levels were found at one campus. Samples will be gathered at five campuses a day, Tuesday through Saturday, now through mid-June.