JEFFERSON CITY, Mo. -- A Missouri teachers union said Friday it is challenging a new measure that restricts teachers' use of social networking sites and their contact with students, saying it violates their constitutional rights.

The Missouri State Teachers Association said it is seeking an injunction to block enforcement of part of a law that takes effect Aug. 28. The union and several public school teachers assert the law violates educators' constitutional rights to free speech, association and religion.

The social networking restrictions are part of a broader law that was proposed after an Associated Press investigation found 87 Missouri teachers had lost their licenses between 2001 and 2005 because of sexual misconduct, some of which involved exchanging explicit online messages with students.

Many teachers have complained the law will hurt their ability to keep in touch with students for classroom purposes, personal problems or even emergencies.

Under the law, school districts must establish policies by January that outline "appropriate use of electronic media such as text messaging and Internet sites for both instructional and personal purposes." Teachers are barred from having "exclusive access" online with current students or former students who are minors. That means communication through Facebook or other sites must be done in public.

The law restricts nonwork-related websites that allow communication between a teacher and a student that cannot be viewed by others, though the measure states it is not attempting to prohibit teachers from setting up nonwork websites that comply with the restrictions.

Spokesman Todd Fuller said the Missouri State Teachers Association has heard from an increasing number of teachers that school districts have interpreted the law in different ways, including some who say that they have been told they cannot have a Facebook page.

The group's lawsuit -- a copy of which was provided to AP -- says the restrictions for nonwork-related sites amounts to prior restraint and violate educators' free speech rights. It also says they could impede religious freedom and association rights by barring teachers from using nonwork-related websites and social networking sites that allow exclusive access with students.

It "is so vague and overbroad that the plaintiffs cannot know with confidence what conduct is permitted and what is prohibited and thereby 'chills' the exercise of first amendment rights of speech, association, religion, collective bargaining and other constitutional rights by school teachers," the lawsuit says.

State Sen. Jane Cunningham, who sponsored the legislation, said critics misunderstand the law. She said teachers are not barred from using Facebook and other websites. They also aren't prohibited from communicating with students, providing discussions are public.

"It only stops hidden communication between an educator and a minor child," said Cunningham, R-Chesterfield.

The law also requires schools to share information about teachers who have sexually abused students with other school districts and allows a lawsuit if a district does not disclose that information and the staff member later abuses someone else. The teachers' group is not challenging that part of the law.

The bill won broad support in the legislature this year and was supported by several education organizations, including the Missouri State Teachers Association. However, the restrictions on communication between teachers and students have received increased attention and have prompted growing concerns from some.

Fuller said he thinks other parts of the legislation received the attention when lawmakers were considering it.

"Everybody was focused on the bill as a whole and really wasn't focused in on the issue of social media and really the scenarios that were going to crop up," Fuller said.

The lawsuit was filed against the state, Gov. Jay Nixon, who signed the legislation into law, and Attorney General Chris Koster, whose office is responsible for representing state government in court. The attorney general's office declined to comment Friday.