PARKLAND, Fla.  — Students at Marjory Stoneman Douglas High and other schools across the U.S. bowed their heads in a moment of silence and took part in volunteer projects Thursday to mark the anniversary of the shooting rampage that claimed 17 lives. But for many Parkland students, the tragedy was still so raw they couldn’t bring themselves to set foot in the building.

Fewer than 300 of the 3,200 students at the high school showed up for what was only a half-day, with classes cut short so that the teenagers would not be there around 2:20 p.m., the traumatic moment last year when gunfire erupted.

Senior Spencer Bloom skipped school to spend the day with students from the history class he was in during the shooting. He said he struggles with panic attacks and feared he might have one if he went in to school.

“There’s all this emotion and it’s all being concentrated back on one day,” Bloom said.

The massacre on Feb. 14, 2018, inflamed the nation’s debate over guns, turned some Parkland students into political activists and gave rise to some of the biggest youth demonstrations since the Vietnam era.