The scandal-hit American School in Japan admitted Friday that it concealed decades of sexual abuse it said was committed by former teacher Jack Moyer and offered its first public apology since the scandal broke in March last year.

"Survivors attempted many times to expose abuse, and we are ashamed to report that they were rebuffed or ignored by the school," the international school's board of directors said in a message to alumni on Friday.

"We apologize for the great harm that was caused by teachers and administrators who failed to protect the students in their charge," the school said, vowing to ensure the "safest possible environment" for present and future students.