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Four high school wrestlers in Oklahoma were charged Monday with raping two other wrestlers — including a 12-year-old boy — on a school bus after a tournament last month.

One of the alleged assailants, Tanner Shipman, 18, was charged as an adult, according to documents filed by prosecutors in Garvin County, south of Oklahoma City. He faces three counts of rape by instrumentation.

The other three, who NBC News is not naming, are 17 and 16, and were charged as youthful offenders. Two are accused of the same crimes as Shipman; one has been charged with a single count.

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The second victim was 16, the documents say.

According to prosecutors, the junior varsity wrestlers were on a school bus returning to Norman after Jan. 9 tournament in Pauls Valley, approximately 40 miles away, when Shipman “restrained” the 16-year-old under a bus seat while two friends sexually assaulted him.

Later, Shipman and others assaulted a 12-year-old — identified in the documents as "Victim B" — at the back of the bus and again in a “secluded area” near Norman North High School after the bus had returned, the documents say.

"When Victim B escapes, Tanner and his friends attempt to do all of this again as Tanner tackles Victim B and forces him between two parked vehicles,” the documents state. “The incident finally ends when Victim B’s parent arrives in the parking lot."

It is unclear if Shipman and the others have lawyers; attempts to reach addresses listed in the charging documents were unsuccessful.

In a statement Monday, Norman Public Schools superintendent Joseph Siano said that “we are well-prepared to handle situations like this and have programs in place to support students who experience trauma. And, while we believe the district’s policies regarding student supervision are strong, we continuously evaluate our procedures to ensure they are followed.”

Siano said the “vast majority” of the student athletes “demonstrate positive behavior and strong character traits.”

After the incident was initially reported, several students were suspended, an adjunct coach who was supervising the students was “immediately” dismissed, and a second coach was suspended, the school said in a statement.