The cleanup at Willingboro High School initially was expected to take about a week. It was closed Sept. 12, after several mold-infested areas were found inside the building.
But the work took much longer than expected, so classes for the school's 1,000 students were moved in late September to another building in the Burlington County district.
School officials say a contractor has provided a "thorough remediation and restoration" of the high school.
Experts say mold problems in the region were likely worsened by this year's record-breaking rains.