Trump's 11-year-old son, Barron, has attended the Potomac school since last year.
"What we do not need is to arm our teachers with guns, which is dangerous and antithetical to our profession as educators," states the letter, which was obtained by the New York Daily News and published as full-page ads in the Baltimore Sun and the Washington Post on Wednesday.
The letter was published the same day thousands of students across the country walked out of their classrooms to protest gun violence in the wake of last month's shooting at a Florida high school.
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Echoing talking points from the National Rifle Association, Trump has argued that armed teachers could prevent school shootings.
Gun control advocates have excoriated Trump over the proposal, saying more guns will only lead to more violence.
Advocates have instead urged Trump to push for beefed up background check systems, more mental health resources and a higher sales age for firearms, among other suggestions.
Kosasky, Barron's principal, agrees.
"Students — children — who have experienced this trauma and loss firsthand are showing the way," the letter says of the Parkland, Fla., students who have been vigorously pushing for gun control since 19-year-old Nikolas Cruz stormed into their high school armed with an AR-15 assault rifle and killed 17 people on Feb. 14.
"We need a robust system of registration and background checks, with a particular eye toward weapons capable of rapidly firing a vast number of deadly shots," the letter added.
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