The deadly shooting at Marjory Stoneman Douglas High School in Florida is the 18th school shooting in the USA in 2018.
Gun campaign group Everytown for Gun Safety said 17 school shootings had happened this year before gunman Nicolas Cruz opened fire at the school 45 miles north of Miami.
The 18 shootings span just 45 days.
Wednesday’s shooting, in which 17 people were killed, is the first this year in Florida, which becomes the 14th US state to experience a school shooting in 2018.
According to Everytown for Gun Safety, no one was injured in eight of the incidents, although a weapon was fired.
Another two were listed as attempted or completed suicides, where no one else was injured.
Cruz, a 19-year-old former student at the school, has been taken into custody.
He has been described by those who know him as a loner who was ‘obsessed’ with guns.
It is thought he is a former member of the US Army Junior Reserve Officers’ Training Corps programme.
The shooting happened at the high school in Parkland, 45 miles north of Miami, at the end of the school day on Wednesday.
Students described hiding under desks and in cupboards as gunshots were heard across campus.
Donald Trump tweeted his condolences shortly after, writing: ‘My prayers and condolences to the families of the victims of the terrible Florida shooting.
‘No child, teacher or anyone else should ever feel unsafe in an American school.
‘My thoughts and prayers are with the students, their families and the entire community.
‘We will continue to receive briefings from law enforcement and issue updates.’
Mr Trump later added: ‘Just spoke to Governor Rick Scott. We are working closely with law enforcement on the terrible Florida school shooting.’
Police said the scene was still ‘very active’ Wednesday afternoon, with reports of parents calling their panicked children who were trapped in the building.
The school, which has 3,100 pupils, was put on ‘Code Red’ lockdown.