AUSTIN, Texas — Texas’ new law allowing concealed handguns in college classrooms, buildings and dorms has barely started and already faces a legal challenge seeking to block it before students return for the fall semester on Aug. 24.

Three professors at the University of Texas sued July 6 to overturn the law, claiming it is unconstitutional and is forcing colleges to impose “dangerously-experimental gun policies.” The 50,000-student Austin campus has been a flashpoint of opposition to the law among faculty and students.

The law took effect Monday, the 50th anniversary of Charles Whitman’s sniper attack from the top of the University of Texas campus clock tower, a shooting spree that claimed 17 lives and has come to be accepted as the nation’s first mass shooting.

U.S. District Judge Lee Yeakel had previously scheduled a preliminary injunction hearing for Thursday in Austin.

Texas has allowed licensed concealed handguns in public since 1995 but had made college buildings off limits.

The new law makes Texas one of eight states with laws that allow weapons on campus and inside buildings. Another 23 let their campuses or governing boards decide.

Texas Attorney General Ken Paxton on Tuesday called the lawsuit “frivolous” and urged its dismissal. Gun rights advocates say it’s a key self-defense measure that is protected under the Second Amendment.

“I’m confident it will be dismissed because the Legislature passed a constitutionally sound law,” Paxton said.

The Texas law allows schools to set some gun limits, such as banning weapons from campus hospitals or labs with dangerous chemicals. The University of Texas rules allow professors to ban weapons from their private offices and places some restrictions on dorms.

The lawsuit by professors Jennifer Lynn Glass, Lisa Moore and Mia Carter says allowing guns into classrooms could be dangerous when discussions can wade into emotional topics.

“Compelling professors at a public university to allow, without any limitation or restriction, students to carry concealed guns in their classrooms chills their First Amendment rights to academic freedom,” their lawsuit says.

Texas’ Republican-majority Legislature passed the law in 2015 over similar objections from student and faculty groups, most notably at the Austin campus.

University of Texas System Chancellor, former Retired Adm. William McRaven, a former Navy SEAL who coordinated the raid that killed Osama bin Laden, urged lawmakers not to pass the law, telling them allowing guns would make campuses less safe.

Supporters of the campus carry law say its impact is overstated, because most students won’t be old enough to legally carry concealed weapons.

Texas law requires handgun license holders to be 21 years old (18 if active military), have clean criminal records and pass classroom and gun range training.