Students mourn during a candlelight vigil for victims of yesterday’s shooting at nearby Marjory Stoneman Douglas High School in Parkland, Fla. on Feb. 15. (Reuters/Carlos Garcia Rawlins)

Sixteen-year-old Colleen Lance’s English class spent much of its time on Thursday talking about what to do in case someone started shooting.

“Do we try to barricade the door? Do we try to escape? What are we supposed do during a shooting?” she and her fellow students asked their substitute teacher.

Lance’s classroom in Illinois was a thousand miles from Marjory Stoneman Douglas High School, where a gunman opened fire on Wednesday and killed at least 17 people. But the news hit close to home.

“After Florida, we were all definitely like, ‘Oh, God, is this going to happen here?’ ” Lance said. “It’s very anxious to go to school after a school shooting. It makes you more aware that you’re not safe.”

Lance wasn’t alone in her anxiety. After the latest school shooting, this time in Parkland, Fla., teenagers across the country returned to their own high schools Thursday for another normal day. But then again, Wednesday had started out as a normal day for Marjory Stoneman Douglas High School. And it was hard not to think about that.

Several students responded to a request from The Washington Post via Snapchat to talk about how the news of the tragedy weighed on them as they returned to their own classrooms. (The Post spoke with some of those students by phone, and contacted their schools to verify their identities and attendance.)

“It makes me feel really concerned. It feels so much more common now,” said Rebecca Connor, 15, who attends high school in Virginia. “You don’t know where it’s going to happen next. It could happen at our school. There’s no way to tell.”

She added that report after report of troubled students turning on their classmates “makes me want to be nicer, makes me want to take everyone’s feelings into account.”

This type of response isn’t limited to high school students, said Robin Gurwitch, a psychologist at Duke University Medical Center. But teenagers are particularly vulnerable.

“With teens, the more stressed they are about these events, the more likely they are to have sleep problems, difficulty concentrating and irritability. It can affect school performance and relationships with peers,” Gurwitch said.

For some, the overwhelming sensation wasn’t fear but weariness.

“It’s become such a common occurrence that it’s hard to have a reaction at some point,” said Aidan McGahey, 15, who attends high school in Philadelphia. McGahey said his fellow students sometimes talk about shootings in the immediate aftermath, and that his religion class had tackled the subject this week.

Lance dissected the news with her friends, who are similarly interested in keeping up with current events and fall on both sides of the political divide. Connor, a budding news junkie who watches “Morning Joe” and keeps up to date with Apple News, confessed that she and her friends didn’t discuss school shooting stories much because it would “make it feel more real.”

Gurwitch said that taking time away from the news was critical for teenagers when a story like the Parkland massacre is dominating headlines. She emphasized activities like reading, sports, listening to music, or other activities that allow people to avoid social media or the news for a set period of time. The American Psychological Association has issued similar guidelines for people of all ages dealing with stress related to school shootings in the news.

It’s not uncommon for schools to have lockdown drills or add security systems to protect students in the event of an attack. But in addition to physical security measures, Gurwitch said, schools should allow discussions about traumatic events “in a smaller group, so if there are concerns, [students] feel comfortable asking a question or commenting on something they may have heard.”

No matter how many safe spaces schools create, the teenagers expressed frustration at the recurring issue of school shootings.

“It’s not something we can sit back and say, ‘it’s a part of our society’. We need to be taking active steps to resolve,” McGahey said. “I think we need to work to stop this.”

“It makes me upset that it’s something I do have to think about. I just want to focus on my grades and graduating and going to college and becoming a young adult,” Lance said.

“I don’t want to think about getting shot when I go to school.”