“In so many regards, we were the first,” she said. “We thought the change was going to be immediate because it was so horrific.”

Now 33, she has spent the majority of her life working to get past that day.

Ms. Strassner is married with four children and lives in Colorado. Over the years, she has been broken anew by other school shootings. She gave up on gun control after the 2012 shooting in Newtown, Conn., when the deaths of 20 children did not lead to effective federal legislation.

“But something about Parkland has been different,” she said. “Those wonderful children have taken a stand, and they have done it with courage and eloquence. They truly have inspired a nation. They’ve inspired a nation and they have inspired me.”

She will be marching in Colorado on Saturday, wearing a class of 2002 Columbine sweatshirt.

“Since Parkland, my boxing gloves are back on,” she said. “I had them off after Sandy Hook. I just have this feeling that we are in a moment where change is possible.”