A young girl in New Jersey was seriously injured on Jan. 7 by a rock thrown at her yeshiva day school bus, as her third grade class was returning from a school trip.

The girl’s skull was fractured after being hit by the rock, according to a letter that Bergen County’s Yeshivat Noam sent to parents on Thursday. “She is alert and stable, but she will require surgery to ensure the injury heals properly,” the co-ed, Modern Orthodox day school in Paramus said in the letter. (JNS sought comment from the school.)

The rock was the size of a baseball, according to reporting in Yeshiva World News, which said that the surgery taking place at Hackensack University Medical Center.