The Irish rockers put on a special performance at Mount Temple Comprehensive School in front of hundreds of students on Friday.
This month, five decades ago, Mullen is said to have pinned a note stating "Drummer seeks musicians to form band" on the school noticeboard.
On Friday he joined Bono, whose real name is Paul Hewson; The Edge, whose real name is David Evans; and Adam Clayton for 30 minutes of music - in homage to when the international music stars used to play on top of the school's boiler house.
They are among the world's best-selling music artists, having sold 170 million records.
It comes as the band promote their new album Carnaval De Luz, which features a duet between Bono and Dolly Parton.
It is the band's 16th album and the first of new material since Songs Of Surrender in 2017.
There was a carnival atmosphere as hundreds of children crowded into an area of open ground beside the school boiler house, while others stuck their heads out of the surrounding classroom windows.
They cheered and screamed while holding up signs for their alma mater, while scores of journalists from the international media also watched on.
News of the unique gig, which was attempted to be held as a closely guarded secret, had clearly broken out beyond the grounds as fans outside the gates - and builders several stories up on a nearby construction site - also sought to catch a glimpse of the excitement.
A major production with several moving cameras captured the performance, which was blasted over a loud sound system across the schoolyard into the surrounding Clontarf.
Taking to the stage after the system played Alice Cooper's Schools Out, Bono grabbed the mic and pointed to the school science lab.
Before launching into I Will Follow, he joked to the school: "We escaped the lab - how's the experiment going for you?"
The quips kept coming with some educational advice before the opening "uno, dos, tres, catorce" countdown for Vertigo, Bono said: "If you're taking Spanish lessons, don't take them from me."
An Irish tricolour waved as the crowd heaved and jumped along - with Bono noting the students were "out of control".
While at times short for words due to the spectacle, the Dubliner said Ireland was an island full of sound and music, adding: "We're so proud to be Irish."
Children cheered "I love you Bono" as the multi-generational audience sang along and took videos and pictures.
On behalf of the band, he thanked them for turning up but noted the children of the "rock 'n roll high school" had "nowhere else to go" during the afternoon gig.
Supported by children on traditional Irish instruments from the Music Generation, an Irish music education program supported by the band, those gathered were also treated to a rendition of One.
A chanted demand of "one more tune" elicited an encore of With Or Without You before the band left the improvised stage of the boiler house - 50 years after coming together.
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