The letter, dated May 3 and printed on White House stationery, was addressed to Yvonne Mason, 61, who retired in 2017. After she made the corrections, she snapped a picture, posted the letter on Facebook and mailed it back to the White House.
"It was a poorly worded missive," she told The New York Times on Sunday.
"Poor writing is not something I abide. If someone is capable of doing better, then they should do better."
Mason, a Democrat who lives in Atlanta, had written to Trump to ask that he visit each family of those who died in the shooting that killed 17 people at a school in Parkland, Florida, in February.
According to the former teacher, the letter she received did not address her concerns.
Instead, it listed a series of actions taken after the shooting, like listening sessions, meetings with lawmakers and the STOP School Violence Act, a bill that would authorise $500 million over 10 years for safety improvements at schools but had no provisions related to guns.
Some of the things Mason wrote in the letter were: "Have y'all tried grammar & style check?"
There was more, but she did not correct everything.
"I did not mention the dangling modifier... I focused mainly on mechanics," Mason told The New York Times.
"Nation" was capitalised, so was "states". She circled both the words.
The letter stood in contrast to other letters she has received from politicians, Mason said.
Senator Lindsey Graham, Republican of South Carolina, sent "beautiful" letters that struck a tone that "makes me more important than him", she said.