At least three dozen students have come down with chickenpox at a private school in North Carolina — nearly one-quarter of the student body — in what health officials call the largest outbreak in the state since the chickenpox vaccine became available more than two decades ago.

The students, who range in age from 4 to 11 years old, attend the Asheville Waldorf School in Asheville, N.C., about 120 miles west of Charlotte. They began falling ill in mid-September, said Dr. Jennifer Mullendore, the medical director for Buncombe County Health & Human Services.

The school has 152 children in nursery school through sixth grade, and one of the state’s highest rates of religious exemptions for vaccination.