In addition, Mr. Runcie said the district was considering using metal-detecting wands at school entrances and installing permanent metal detectors — a safety measure Mr. Runcie recently criticized as ineffective. A person intent on committing an atrocity would find his or her way around them, he said in an interview last month.

“Someone is not going to go through a metal detector with an AR-15,” he said at the time, adding that metal detectors do not help create a welcoming learning environment and pose a logistical challenge in a school as large as Stoneman Douglas High, which has more than 3,200 students.

Stoneman Douglas High parents, already on edge since the massacre, became alarmed Monday when the shooting suspect’s brother was arrested on a charge of trespassing, after Broward County sheriff’s deputies said they saw him skateboarding onto the campus after school let out. A deputy assigned to patrol the campus was found asleep by a student the same afternoon, the sheriff’s office said; he was suspended without pay.

Some parents kept their children home on Tuesday, according to Sarahnell Murphy, an assistant state attorney prosecuting the trespassing case against Zachary Cruz, the brother of the shooting suspect, Nikolas Cruz.

Also on Tuesday, the sheriff’s office said it had arrested three Stoneman Douglas High students, one for making threats on Snapchat and two for wielding knives. Jordan Salter, a girl accused of pulling a knife on a boy after a cafeteria confrontation, made a court appearance on Wednesday. Her father, Scott, told the court his daughter had been “terrified” of going to school.