The massacre at Sandy Hook Elementary raised fresh questions about what can be done to protect schools from gun violence more than a decade after the Columbine shootings led schools nationwide to adopt new safety measures.

The 1999 incident at Colorado's Columbine High School—in which two boys killed 12 students and a teacher before taking their own lives—was the deadliest shooting at a U.S. elementary or high school before Friday.

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Many schools responded with measures like installing metal detectors, developing crisis plans, and keeping doors locked and accessible only by buzzer.

Michael Dorn, executive director of Safe Havens International, a nonprofit that works with thousands of U.S. schools to develop safety plans, said there has been a dramatic improvement in school safety since Columbine, "but so much more" could be done. He said that all school staff nationwide should be trained on how to recognize and deal with potential threats. Only some schools have done so.

"There is no strategy that guarantees you can stop school violence, but we need to empower people to feel like they are prepared to deal with potential threats," he said.

After Columbine, many schools nationwide adopted zero-tolerance policies that punished students who brought weapons to school, even if by accident. Those policies led to criticism after students were punished for bringing items, such as nail clippers, to school.

Experts said there is a limit to what schools can do to prevent a determined killer in a nation with easy access to guns.

Officials at Sandy Hook Elementary sent a letter home to parents earlier this year alerting them to a new security system. Under the system, the school's doors were kept locked and every visitor had to ring a doorbell, which was monitored by the school staff. Once inside, visitors had to stop at the office and sign in.

Staff at Sandy Hook had practiced safety drills each semester, and were trained to lock the door and find a safe place in the room in the event of an attack, said Carrie Usher, a fourth-grade teacher there. She said the death toll could have been much higher if not for that training.

"We just heard so much gunfire. It was unbelievable," Ms. Usher said. "It felt like it lasted for five minutes. It wouldn't stop."

Schools already are protected by federal laws that prohibit guns within 1,000 feet of elementary or high schools, with a few exceptions, such as for law enforcement officers. Numerous state laws also protect schools.

As a result, schools are generally relatively safe from gun violence. Each year, since 1992, less than 2% of all youth homicides occurred at an elementary or high school or as children walked to school, according to a joint report issued earlier this year by the U.S. Department of Education and the U.S. Department of Justice.

The number of youth homicides has bounced around during that time, peaking at 34 during the 1992-93 and 1997-98 school years. During the 2009-10 school year, the latest data available, 17 children were killed at—or on their way to—school, according to the report.

Jon S. Vernick, co-director of the nonpartisan Johns Hopkins Center for Gun Policy and Research, said that, after some mass shootings, some states have adopted some stricter gun rules, but said they have not done enough, adding that schools can only do so much. "We can talk about metal detectors all we want, but the best way to make our schools safe is to make our communities safe," he said.

Some have suggested that school security officers—or even teachers—should be armed to help deter potential violence.

—Jack Nicas contributed to this article.

Write to Stephanie Banchero at and Sophia Hollander at