The Tonight Show host offered similar praise, saying, "You are not just the future — you are the present. Keep changing the world. Keep making us proud."
In a video of his address, Fallon joked that the students "won't be classmates anymore. You'll be adults who will Facebook search each other at two in the morning for the next 10 years."
Turning more serious, he said: "First thing is this: When something feels hard, remember that it gets better. Choose to move forward. Don't let anything stop you." He thanked them for their bravery and activism.
The private ceremony for the nearly 800 members of the Marjory Stoneman Douglas High School class of 2018 was held at the BB&T Center, where the National Hockey League's Florida Panthers play. It was moved to the arena to accommodate the expected large crowd. Reporters were not permitted inside the arena.
Fourteen students and three staff members died in the Feb. 14 attack in Parkland. Former student Nikolas Cruz is charged with their deaths and the wounding of 17 other people. Attorneys for the 19-year-old have said he will plead guilty in exchange for a sentence of life without parole. Prosecutors are seeking the death penalty.
The school planned to present diplomas to the families of Nicholas Dworet, Joaquin Oliver, Meadow Pollack and Carmen Schentrup.
Pollack's brother, boyfriend and cousins were to accept her diploma. Her brother, Hunter Pollack, aired his feelings on Twitter.
"Today is the day my sister has been waiting for. Graduation where she would've been getting her diploma and be on her way to attend college. This is a sad day, as I will be walking stage to get her diploma for her," he said.
Her father, Andrew Pollack, said he's too emotionally spent to attend the ceremony. He has been an outspoken critic of school and law enforcement officials, saying they failed to protect his daughter and the others, but that's not why he's staying away.
"It has nothing to do with them," Pollack told The Associated Press by phone Sunday. "I've just been dead inside since Feb. 14."
Instead, he's headed to central Florida, where this week he will address the armed guards one district has hired for its schools.
April Schentrup, mother of Carmen Schentrup, posted a photo of her daughter wearing her graduation gown and cap, the Sun Sentinel reported.
"For me, it is too painful to celebrate w/o Carmen," she said in an online post. "But I am proud of Carmen's friends & classmates on their accomplishments. They've overcome so much. I know they will cont to make positive changes."
As families arrived for the ceremony, gunshots from a nearby public shooting range could be heard echoing over the parking lot.