The Glen Rock schools had been on spring break between March 29 and April 2.
Charleston said the district's contact tracers found that it was "highly likely that each of the students were exposed to COVID-19 outside of the school.
"We must all do our part to ensure the health and safety of our students, staff and community," Charleston said. "Our actions today can have a positive impact on school, activities, spring sports, events and milestone evens, such as prom and graduation, for the remainder of the school year."
There is no change in schedule for the district's remaining schools and the letter cautioned parents and students that "this is not an invitation for students to spend time with each other as we have determined with confidence that COVID-19 is spreading outside of our schools in non-school related social settings."