LAS VEGAS (AP) — Kaitlynn Willoughby was frustrated by how quickly gun control debates stalled after the worst mass shooting in modern U.S. history.

Her friend, 20-year-old Quinton Robbins, was among the 58 people killed Oct. 1 at a country music festival on the Las Vegas Strip. By the holidays, just weeks later, calls for change dissipated.

"No one was listening to us yet," the 18-year-old high school senior said. "After Las Vegas, it was just another shooting."

Willoughby is among those planning to march this weekend in rallies worldwide inspired by high school students in Parkland, Florida,...