Speaking at an event at the White House, Trump said that he wanted to start by "expressing our sadness and heartbreak over the deadly shooting."
At least 10 were killed and others injured at Santa Fe High School in southern Texas. The investigation is ongoing.
"We're closely monitoring the situation and federal authorities are coordinating with local officials. This has been going on for too long in this country. Too many years. Too many decades," Trump said.
"We grieve for the terrible loss of life, and send our support and love to everyone affected by this absolutely horrific attack. To the students, families, teachers, and personnel at Santa Fe High: we're with you in this tragic hour, and we will be with you forever," he said.
He called it an "absolutely horrific attack" and told the families of the victims and the injured "we're with you in this tragic hour and we will be with you forever."
"Everyone must work together at every level of government to keep our children safe. May God heal the injured and may God comfort the wounded," Trump said.
His statements came after he first tweeted about the shooting, writing, "School shooting in Texas. Early reports not looking good. God bless all!"
Vice President Mike Pence, who was at the same event with Trump, also offered his condolences to the community impacted by the shooting.
"We're with you. You are in our prayers and I know you are in the prayers of the American people," he said.
First lady Melania Trump, who has spent the week at Walter Reed National Military Medical Center being treated for a kidney issue, posted a tweet saying that her "heart goes out to Santa Fe and all of Texas today."
Parkland students speak out
Some of the student activists from Parkland High School, who gained national prominence for their gun reform work in the wake of a deadly shooting at their school in February, also tweeted their support.
Delaney Tarr, one of the Parkland students, wrote: "I should be celebrating my last day of high school, but instead my heart is broken to hear of the tragedy at Santa Fe. We cannot let this continue to be the norm. We cannot."
Emma Gonzalez, another well-known student activist, addressed her tweet directly to the students at the school.
"Santa Fe High, you didn’t deserve this. You deserve peace all your lives, not just after a tombstone saying that is put over you. You deserve more than Thoughts and Prayers, and after supporting us by walking out we will be there to support you by raising up your voices," Gonzalez wrote.
Without directly mentioning the shooting, David Hogg wrote: "We are fighting for you."
He also seemed to reference his own experience in the wake of his school's shooting.
"Get ready for two weeks of media coverage of politicians acting like they give a s--- when in reality they just want to boost their approval ratings before midterms," he wrote in a second tweet.
Reaching a national audience
Current and former politicians were also among those to extend their condolences and issue calls for action.
Hillary Clinton, who advocated for gun reform during her 2016 presidential bid, shared her thoughts on Twitter this afternoon.
"Every day that we fail to act on gun violence, we are failing our children. This should be a day for soul searching across America - and that soul searching should be matched with legislating to begin dealing with this national shame," she wrote.
Senate Majority Leader Sen. Mitch McConnell echoed Pence's earlier statements and thanked first responders.
"The prayers of the entire Senate are with the victims of today’s violence in Santa Fe, their families, and the community that is coming together to support them. We are especially grateful for the first responders and others who work to protect and save lives," he wrote.
New York Gov. Andrew Cuomo issued an open letter addressed to Trump and elected officials urging them to act.
"When is enough enough? How many more innocent people have to die before you act? You were elected to lead - do something. Your first responsibility is to the people of this country, not the NRA - do something. My heart breaks for the families who have to grieve from this needless violence - DO SOMETHING," he wrote.
Former Congresswoman Gabby Giffords, who was left with a severe brain injury following a 2012 shooting, issued a statement decrying how today's shooting was another instance where "students stood before national cameras and detailed the terror that unfolded in their school."
"I will not stand for this and neither should you. Parents should not have to hug their children in the morning and worry about whether they’ll see them at the end of the day. We don’t have to live in a country where the politicians let this happen again and again and again," she wrote.
Giffords, who co-founded an eponymous gun violence prevention group, said that "it's time for America to find the courage to take on the powerful and fight for our own safety."
"We need the courage to take on the special interests who say that nothing can be done, that these acts of evil are beyond our ability to control. They are not. The gun lobby and the politicians who receive the NRA’s checks must understand we will not tolerate another vote against our safety," she said in the statement. "Congress must find the courage to pass the effective laws that can protect our children and stop dangerous people from accessing guns. And if this Congress won’t protect our kids and communities from gun violence, this November we will vote in a Congress that will."