One pupil from Tredegar Comprehensive School was on the phone to her mother when police shot an armed man as he tried to enter the US Capitol Building visitor centre in Washington.
Lisa Reardon told BBC Radio Wales how her daughter, Chelsea, began screaming down the phone, "he's got a gun".
Police called the shooting an "isolated" incident.
Ms Reardon paid tribute to the way teachers dealt with children, including her 16-year-old daughter.
"The next minute she is screaming down the phone to me saying, 'he's got a gun, he's got a gun'," she said.
"The first thing that goes through my head is what the hell is going on.
"Even though I'm panicking, I'm trying to calm her down as well, saying, you have to listen to the teacher.
"Everything was going through my mind. I felt more useless than anything."
A female bystander suffered minor injuries when police fired at the suspect.
Ms Reardon said: "The teachers were amazing for looking after them and reassuring them. They are carrying on with their trip. It is not going to stop them.
"I can't wait for her to come home on Thursday."
The school posted a message on Twitter to say all children were fine and shared photos on their Facebook page saying "all are safe and well".
School children from Blaenau Gwent were caught up in a dramatic armed incident while on a trip in the United States.