Highlands Ranch, Colo. — A shooting Tuesday at STEM School Highlands Ranch is just a few miles from Columbine High School, the scene of one of the worst school massacres in U.S. history. Since then, an estimated 228,000 American students have experienced gun violence at their school.

After parents rushed to be reunited with their children, they learned about their school day horror.

"We heard really big bangs which I assume are gunshots," said student Mia Bash.

Taami Bash was texting her daughter during the attack, finding out it wasn't just a drill. But the days ahead could be some of the hardest, as survivors begin to cope with the psychological impact of the trauma they witnessed.

A recent study by the Giffords Law Center to Prevent Gun Violence found that 40 percent of children who experience gun violence suffer from post-traumatic stress disorder (PTSD). Sixty percent of high school students are afraid of a shooting occurring in their school or community.

"There stages of trauma. they're in shock that our bodies go into a protective mode so that we can cope with it and get by," said Robin Finegan, a victims advocate who has been to scenes like this before.

Finegan said it can take a long time to recover.

"Everybody comes into their trauma and tragedy with everything that's in their life currently and all those factors contribute to their resilience," she said.