Extra police officers were sent to the high school to provide security before the student was taken into custody.
Plano police said the juvenile male was arrested without incident on campus and the gun was confiscated. The former student, Nikolas Cruz, was charged with 17 counts of premeditated murder.
Police say 18-year-old Alwin Chen of Germantown brought a handgun to Clarksburg High School Thursday.
The letter says the student didn't appear to have any intentions to hurt or threaten anyone.
Captain Paul Starks, a police department spokesman, said the gun was in the student's backpack. "Due to their quick response to a suspicious situation, we were able to ensure the safety of our campus", Shafferman said in his statement.
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In a series of Thursday tweets, police said the school resource officer was informed that a student was concealing a weapon.
In an effort to keep you informed, and especially in light of the news that is on the hearts and minds of our national community, I am writing to make you aware of a situation that we managed at our campus today. School administration and our school resource officer immediately acted on this tip and made contact with the student.
"All safety procedures were appropriately followed according to the school's safety plan, " he said.
It also said the process worked in keeping their students safe. His name has not been released to the media. As always, we appreciate your efforts in monitoring your children's friends, activities and free time, and by ensuring that they do not have access to weapons.