The Delaware County District Attorney's Office announced Thursday that it was dropping charges filed last week against the headmaster of a Main Line school who was accused of assaulting his teenage son after finding marijuana in the boy's backpack.

In a one-paragraph statement, District Attorney John J. Whelan said that while police had probable cause to arrest Haverford School headmaster John Nagl, they had "insufficient evidence" to pursue prosecution.

Whelan's announcement did not immediately assure the reinstatement of Nagl as headmaster, a position he has held for more than three years.

Last week, the school's board of trustees placed Nagl on paid administrative leave after the charges were filed. On Thursday, William C. Yoh, the board chairman, said in a statement that the board would meet next week to "fairly and deliberately assess the situation in light of this new development."

Nagl on Thursday was "relieved" but "still concerned for his child," according to his attorney, Robert Keller.

"It's his desire to continue as the headmaster, and he's hoping to be able to return as quickly as possible," Keller said.

The Thursday announcement, Keller said, came as a result of a Wednesday meeting between Nagl and Whelan. In the end, Keller said, "the facts showed that the parent took appropriate steps to discipline a child who was acting inappropriately."

"The father acted within the statutory bounds of the law in disciplining his child, and that was clearly demonstrated to the DA," he said.

The incident involving Nagl's arrest came as a result of an Oct. 10 altercation with his son in their Haverford home, court documents show.

According to those documents, two days prior to Nagl's arrest he found marijuana in his son's backpack. As punishment, he confiscated the marijuana, along with his son's cellphone.

Two days later, after noticing that the marijuana and phone he had hidden were removed, Nagl, 50, allegedly overheard his 15-year-old son using the phone in a bathroom of their home. Nagl attempted to take the phone back, but his son refused, the documents allege, forcing Nagl to take it from his son's hands.

At that point, as Nagl walked away, his son "reengaged in a physical altercation over the phone." Nagl, according to the documents, told police "he put his son in a choke hold to end the altercation quickly."

Nagl's son called 911, saying he felt "light-headed." When police arrived, they found a "visible red mark on [his son's] chest below the neck line," according to the documents.

That day, Nagl was arrested and charged with simple assault - a third-degree misdemeanor - and released on $30,000 unsecured bail.

Through Keller, Nagl declined to comment.

Prior to Nagl's becoming headmaster, his LinkedIn account shows, he served as an Army officer with combat experience in Operation Desert Storm during the Gulf War and Operation Iraqi Freedom. An expert in counterinsurgency, he earned his master's and doctorate degrees from Oxford University, where he studied as a Rhodes scholar, according to the Haverford School website.

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