The incident began around 1:40 p.m., Tuesday, Dec. 3, after the alarm went off.
Officers from the Dutchess County Sheriff's Office and the State Police responded en mass and secured the school before conducting a search.
Dutchess County Sheriff's Capt. John Watterson said there was "no threat," and that all inquiries should be directed to the school district.
The district, which had earlier sent out a message to parents telling them to stay away, said that school will be dismissed around 4:15 p.m.
Students on smaller vehicles such as District vans will depart around 4:30/4:45 p.m.
Parents waiting at LaGrange Middle School to pick up students may begin to pick up children at Arlington High School at 4:30 p.m.
The Dutchess County Crisis Team will be available at Arlington High School on Tuesday afternoon in the Library.
"We encourage all staff, students and family members that need support to utilize this resource," the school said on its website.
All AHS after-school activities have been canceled.
The district is looking into why the alarm was triggered.
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