Mason Severson, 27, a former student of Mr. Baish’s, asked in an Oct. 2 Facebook post who was going to attend a climate strike rally in Iowa City on Friday, and linked to an article with a photo of Ms. Thunberg.

Mr. Baish responded, “dont have my sniper rifle,” according to the post.

Mr. Severson said on Sunday said the post “was insensitive and taken too far.”

“It wasn’t a joke,” said Mr. Severson, who is a pre-med student at the University of Northern Iowa. “It wasn’t baseless. It was irrefutably vile and wrong.”

Mr. Baish teaches chemistry at West High School in Waterloo, Iowa, about 55 miles north of Cedar Rapids.

Mr. Baish, school officials and the Waterloo Education Association, the teachers’ union, could not be reached for comment on Sunday.