(CNN) -- Roughly half the students of a small Texas school district were out of classes Tuesday after an anonymous writer threatened to kill students unless "bleeding heart liberals" cracked down on discipline, the superintendent said.

School officials in Bay City, about 80 miles south of Houston, canceled outdoor activities, changed bus routes and posted higher-profile guards in the city's eight schools Tuesday -- but Superintendent Keith Brown, whose family was personally threatened in the letter, said classes would continue as scheduled for the week.

Bay City Independent School District published the threatening letter Monday in hopes that the public would help authorities catch the sender.

"We don't allow terrorism in Matagorda County, and we're going to fight this and have our community members help us solve the crime," Brown said.

Nevertheless, he said, about half of the district's nearly 3,800 students stayed home Tuesday.

The letter, riddled with profanity and misspellings, demands that Bay City schools enforce quiet in their classrooms and stiffen detention and suspension policies. The author, who claims to be facing a terminal illness, said his or her child was attacked after telling "some kidds to shut up in class one day."

"You (expletive) bleeding heart Liberals have let the pice of (expletive) kidds run your schools. Now here are my rules. Break one and see what happens," reads the letter, received December 28.

The writer threatens to kill a random student if administrators ignore the demands, because Brown's own children "will be well garded by then."

"Your lack of student control has hurt my child and I am termanal and will only be with them for 18 months," the letter continues. "School will not be a nightmare for them."

The handwritten letter had a Bay City postmark, but a fictitious return address, Brown said. It was sent to state police in Houston, who are assisting in the investigation, along with an FBI agent, he said.

"This person chose the wrong avenue to direct their attention," The superintendent said. "They made a very poor choice. What they need to do is come forward, so they can get help in dealing with what the issue might be."

In a later interview, Brown told CNN his own daughters attended class on Tuesday, and were "just as safe as all the rest of our students in all our schools."

Brown told HLN's "Prime News" that officials believed parents should know about the letter.

"We felt ... that transparency would be the best [approach] in trying to utilize our whole community to identify this terrorist," he said.

Tammy McDonald, whose daughter attends Bay City's high school, said parents received phone messages from the district Monday night, directing them to the district's website for details of the threat. She compared the response to the days following the September 11, 2001, terrorist attacks on New York and Washington, with parents resolving that they won't be "pushed around."

"We are nervous, of course. We want our children to be safe," said McDonald, president of the high school band boosters. But she added, "Our children are going to continue to go to school."

"Even if (the letter-writer) had good ideas about changing the school, it will definitely not change now because of the way they handled this," she said.

But Brown said about 50 percent of students were absent Tuesday. He said the district would consider that "a personal choice as a parent" -- but that the students would be marked absent unless the district obtains a waiver from state officials. Bay City will seek that waiver, he said.

CNN's Matt Smith and Sara Pratley contributed to this report.