from Wicked Local Milton

The high school was in lock down for nearly four hours Monday after a parent found a shotgun shell outside the main entrance.
Deputy Police Chief Charles Paris said officers using specially trained dogs from Quincy and Boston police were unable to uncover any weapons or ammunition during a search of classrooms, lockers, common areas and student backpacks. (Read the full story here.)

and The Boston Globe – Milton High locked down after shotgun round found near entrance