At Princess Anne High School in Virginia Beach, 420 students are signed up on the After School app, an anonymous message board that is closed to school officials, parents and other grown-ups.

School district Superintendent Aaron Spence had not even heard of the app until in the past few days, when a high school student posted an anonymous threat to one of the app’s message boards, which a parent brought to the attention of police Tuesday evening. By Wednesday, the student, a 17-year-old, was in police custody and charged as a juvenile making threats against the school.

A school spokeswoman said the anonymous post made a reference to a student allowing certain people to leave the school “before I start shooting.”

After the student’s arrest — and in response to a threat posted last week on Instagram that led to a police investigation at Virginia Beach’s Green Run High School — Spence wrote to parents in the Virginia Beach City Public Schools system telling them to educate their children about the pitfalls of using social media.

“I come at this from the educational perspective,” Spence said. “We want to teach our students how to be responsible online citizens and create a positive digital profile.”

The After School app is on more than 22,300 U.S. high school campuses and offers a space in which high school students exclusively can share their thoughts without revealing who they are. Although its founders said they created the app to give teens a forum for discussing sensitive issues, many students have said it is an avenue for cyberbullying, harassment and threats of the kind seen in the past week in Virginia Beach.

Like administrators across the country, Spence is grappling with the challenge of effectively monitoring students’ use of social media while respecting students’ rights to expression but also taking threats seriously.

The arrest in Virginia Beach was at least the second in the country in the past week related to the app. On Monday, an 18-year-old in Warren, Mass., was charged with making a threat on the app that put his high school on alert and kept many students away from classes amid fears of an attack. Last month, at Lee High School in Springfield, Va., extra police were sent to the school after an anonymous post of a photograph of a gun and a mention of something “going down” was posted on the app.

Last year, before the creators upgraded safeguards in the app, a teenager in Michigan wrote a series of frightening posts threatening to carrying out a Columbine-style shooting and talking about using his classmates for target practice. He pleaded guilty to making the threats and was sentenced to 90 days in jail.

The posting of threats continues despite the addition of moderators to monitor everything posted to After School around the clock, according to the app’s creators. Objectionable posts — including threats, taunts and the use of profanity — are supposed to be blocked from the message boards, which are tied to individual high schools and cannot be accessed by parents or administrators.

In some cases, a user has tapped out a threat on his phone and then posted it. Even if it’s blocked by moderators, however, it will appear in the user’s personal message board. From there, the user can take a screen shot and make it appear that it was posted to the board. The app’s designers also created an instant alert system to notify superintendents and police if a threat is posted to a local message board.

Spence, the Virginia Beach schools superintendent, said he was never contacted by After School’s administrators. Master Police Officer Tonya Pierce, a spokeswoman for the Virginia Beach Police Department, said investigators worked with the app to identify the student. After School officials did not immediately respond to a request for comment.

Spence has not advised students to stay off the app, and he said he is keenly aware that even without that app, students still have avenues for placing anonymous posts on social media. But he said he wants them to know that serious legal consequences can arise from what they write, even if their posts are made with no ill-intent and they think they are posting anonymously.

“We all live in an era of heightened anxiety and heightened awareness,” Spence said. “These kind of threats won’t be tolerated.”