But in a session that began as a subdued conversation and sometimes descended into tears and shouting, policy proposals were overshadowed by raw expressions of fear, anger and sorrow.

“We’re here because my daughter has no voice — she was murdered last week, and she was taken from us, shot nine times,” said Andrew Pollack, whose daughter Meadow was one of the 17 killed in Parkland. “How many schools, how many children have to get shot? It stops here, with this administration and me, because I’m not going to sleep until it’s fixed.”

Most of the students and parents invited from the Florida school appeared to support Mr. Trump, many of them prefacing their comments with praise for his leadership. But even fans of the president vented anger and desperation, laying the challenge of responding to the tragedy at his feet.

“It should have been one school shooting, and we should have fixed it — and I’m pissed,” said Mr. Pollack, the only parent of a child killed in Parkland who was at the session, raising his voice as he looked at Mr. Trump. “Because my daughter, I’m not going to see again.”

Samuel Zeif, 18, told of texting his parents and brothers from the second floor of Stoneman Douglas, believing that he would be killed, and he dissolved into tears as he begged the president, “Let’s never let this happen again — please, please.”

“I don’t understand why I can still go in a store and buy a weapon of war, an A-R,” Mr. Zeif said, referring to the AR-15 rifle. “How is it that easy to buy this type of weapon? How do we not stop this after Columbine, after Sandy Hook? I’m sitting with a mother who lost her son. It’s still happening.”