A Michigan school district canceled a proposed optional lesson for first graders about novel gender pronouns, including "tree" and "ze," citing security concerns.

"The goal of the voluntary mini-lesson was to help promote Dewitt Public Schools' vision of a safe, nurturing, and supportive learning environment where all learners can succeed," DeWitt Public Schools Superintendent Dr. Shanna Spickard wrote in a message on April 19 to the district's families and staff. "Unfortunately, it has become a major disruption and distraction to that vision in which our staff, administrators, and students feel unsafe."

Schavey Road Elementary School in DeWitt, Michigan, had sent a letter on April 11 informing parents of the school’s plans through the optional lesson to "help students share and explore pronouns through discussion and literature to embrace differences and promote acceptance."

Following the announcement, Spickard said staff members had received inappropriate, angry and threatening phone calls, emails and social media messages, and the lesson was canceled for safety reasons.

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"While the vast majority of these inappropriate communications have originated outside of our community, several staff members have expressed feeling anxious, stressed, and even afraid to go to school," she wrote. "This is unacceptable. We are in contact with local law enforcement regarding these communications and have increased both police and administrative presence as precautionary measures."

Spickard told Fox News Digital that she did not have any additional comment or information.

Republican House candidate Tom Barrett, who's running in Michigan's 7th Congressional District, which includes the school, and had spoken out against the lesson, expressed gratitude in a statement.

"My wife and I have four young kids, so it was shocking to learn that a local school was teaching alternative pronouns to first graders right here in mid-Michigan," he told Fox News Digital. "It’s already incredibly difficult to protect our kids from inappropriate content, so the last place we should have to worry about them is in elementary school. While I’m grateful that common sense prevailed and this lesson was canceled, it was an unnecessarily close call that is becoming far too common."

The proposed lesson planned to use the book "They She He Me: Free to Be!" by Maya Gonzalez, which includes cartoon pictures of men and women using interchangeable pronouns and teaches kids to embrace nonbinary gender terminology, including "ze" and "tree." 

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"On the inside, you may not feel like a he or she at all," the book reads. "Maybe they feels most free or you may feel like both she and he." Children are taught they can "claim" their pronouns outside "he and she" by creating the way they want to be addressed. 

"You can use your own name as a pronoun," the book says. "You can change pronouns from he to she or she to he. You can use new ones like ze or create your own like tree! Some people use they, which is a perfect way. There are many more pronouns waiting to be discovered and used." 

The book also instructs kids to use the pronoun they observe a playmate saying and if they do not hear a pronoun, the child can always use "they." Children are also instructed to "play with pronouns" by imagining how characters feel on the inside and changing their pronouns. 

Spickard said the goal of the lesson was to help promote a safe, nurturing and supportive learning environment where all learners can succeed. She added she and the school board did not make the decision lightly but ultimately elected to listen to staffers' safety concerns.

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Before the lesson was canceled, Spickard released a statement to Fox News Digital that said the "optional mini-lesson is not part of the core curriculum but is a supportive measure to promote inclusivity, a core value of our district."

Fox News' Hannah Grossman and Maria Lencki contributed to this report.