* Editor's note: This story has been updated throughout.

Texas Attorney General Ken Paxton on Tuesday called a lawsuit brought by three University of Texas at Austin professors against the state’s campus carry law “frivolous” and said the professors have no valid reasons for opposing guns on campus.

Paxton filed a brief in response to the lawsuit Monday, saying the professors' request to block the law — which went into effect Monday — before the first day of fall classes is unconstitutional.

“It is a frivolous lawsuit, and I’m confident it will be dismissed because the Legislature passed a constitutionally-sound law,” Paxton said in a statement. “There is no legal justification to deny licensed, law-abiding citizens on campus the same measure of personal protection they are entitled to elsewhere in Texas.”

UT Austin professors Jennifer Lynn Glass, Lisa Moore and Mia Carter filed a lawsuit opposing the state’s campus carry law last month. They asked a federal court judge to grant an injunction that would block students from carrying guns into university buildings before the first day of class, which will be Aug. 24. Paxton's brief opposes the request for an injunction, saying it would be unlawful to ban concealed carry of guns on campus. A hearing on the case is scheduled for Thursday.

Read More Three UT Professors Sue to Block Campus Carry Law

The law was approved last year and went into effect Monday, making Texas the eighth state in the nation to allow campus carry. The law allows concealed handgun license holders to carry their weapons inside most buildings and classrooms on public university campuses, though some limited restrictions apply. UT Austin professors can ban guns from their private offices, the university system's regents decided last month. Private universities may opt out of the law, and all but one have chosen to do so.

The professors claimed the law is forcing the school to impose "overly-solicitous, dangerously-experimental gun policies" that violate the First and Second Amendments. In their suit, professors say they teach courses that touch on emotional issues like gay rights and abortion. The possibility of guns on campus could stifle class discussion, which is a violation of the First Amendment, the suit says.

The professors also claim the law violates the 14th Amendment, which promises equal protection under the law, by holding public and private universities to a different standard.

“ It is a frivolous lawsuit and I’m confident it will be dismissed because the Legislature passed a constitutionally-sound law. ” — Attorney General Ken Paxton

In his response, Paxton says the professors’ arguments do not justify banning concealed carry on campus. They have no individual right to academic freedom under the First Amendment, the brief says. It goes on to say that it is irrational to claim the state cannot treat public and private institutions differently, as it does this in countless other areas of the law.

“Plaintiffs will not be irreparably harmed if a preliminary injunction is issued, but Defendants will be,” the brief says. "The citizens of this state — and in particular the students who wish to take the classes offered by Plaintiffs — will be denied both their statutory and constitutional rights.”

UT also filed a brief responding to the professors’ lawsuit, in which it claims there is no evidence to support banning guns in classrooms.

When it comes to limiting robust discussion in the classroom, “plaintiffs do not allege more than subjective chill,” the brief says.

Since the law is already in effect, the professors would have to present a very strong argument to make an exception for them, the brief adds.

Renea Hicks, a lawyer representing the professors, said Paxton and UT are undermining the real threat to his clients’ academic freedoms.

“It isn’t a vague fear that something might happen,” Hicks said. “It’s the palpable certainty that there will be narrowed spectrum of academic debate because of guns in the classroom.”

Disclosure: The University of Texas at Austin has been a financial supporter of The Texas Tribune. A complete list of Tribune donors and sponsors can be viewed here.