Parkland high school honors 'those not with us' at first graduation since 17 killed in massacre The senior class of Marjory Stoneman Douglas High School graduated Sunday.

Nearly four months after a mass shooting killed 17 people at Marjory Stoneman Douglas High School, the class of 2018 graduated and posthumous diplomas were presented to families of the four seniors who lost their lives.

Hundreds of seniors from the high school in Parkland, Florida, were clad in sashes that read "MSD Strong" as they walked across the stage to accept their diplomas during a private graduation ceremony Sunday afternoon at the BB&T Center in nearby Sunrise.

The ordinarily joyous occasion was bittersweet. Among the those killed in the Feb. 14 massacre were several 12th-graders who would have been graduating alongside their classmates: Nicholas Dworet, Joaquin Oliver, Meadow Pollack and Carmen Schentrup.

Each were honored with posthumous diplomas during Sunday's ceremonies.

Principal Ty Thompson welcomed the crowd at the school’s 27th commencement ceremony, saying the commencement would “honor those not with us,” reminisce on the past four years, and celebrate their accomplishments.

The event's program included the aspirational message: "Be the change you wish to see in the world."

Senior Eagles, Stoneman High School's mascot, wore a special sash emblazoned with the rallying cry "MSD Strong" along with their caps and gowns.

Some family members of the victims did not attend, telling ABC News station WPLG that it was too painful.

"For me, it is too painful to celebrate without Carmen," her sister, April Schentrup, told the station. "But I am proud of Carmen’s friends and classmates on their accomplishments. They’ve overcome so much. I know they will continue to make positive changes."

Others attended in place of some of the victims.

"I did not get to walk with my graduating class, but today I had the privilege to walk the stage in honor of my dead sister," Meadow Pollack's brother, Hunter, posted on Twitter. "Hope she’s proud!"

Manuel and Patricia Oliver, the parents of Joaquin Oliver, accepted their late son’s diploma on his behalf.

Patricia Oliver wore a bright yellow shirt under a vest that read: “This should be my son.”

They said they never imagined they would see their 17-year-old son's classmates graduating without him.

"We had a lot of plans for our lives," Manuel Oliver told ABC's "Nightline" in an interview Friday. "These kinds of events destroy any plan that you had and leave you in an empty space with no plan at all."

The crowd in the arena applauded as she hoisted a framed diploma and cap over her head.

The graduation took place as the school's alleged gunman, Nikolas Cruz, a 19-year-old former student, remained incarcerated after being charged in connection with the 17 deaths as well as the wounding of 17 others. If convicted, Cruz could be sentenced to the death penalty.

The teen's defense attorneys have said he would plead guilty "in exchange for a waiver of the death sentence."

At the graduation, "Tonight Show" host Jimmy Fallon delivered a surprise commencement speech. He lightened the otherwise somber mood with jokes teenagers could relate to.

"You won't be classmates anymore. You'll be adults who Facebook search each other at 2:00 in the morning for the next 10 years," Fallon quipped.

The comedian also praised the students' resolve in their March for Our Lives protest back in March.

"You took something horrific and instead of letting it stop ... you started a movement," he told the students. "Not just here in Florida, not just in America, but throughout the whole world. The whole world has heard your voice, and that was you making a choice."

Since the mass school shooting, Stoneman's students have seized on the watershed moment to led a movement to push for tougher legislation on gun control.

More marches are expected as the movement evolves.

"Huge announcement tomorrow. We’re so excited to let the world know what we’ve been working on the past few months. Get ready! #MarchForOurLives," junior Jaclyn Corin posted on her Twitter account.

ABC News' Stephanie Wash contributed to this report.