"The first [concert] I really, like, actively wanted to see was High School Musical live," Jackson told Fallon. "[Zac] actually wasn't there. I was so bummed."
"He was filming that movie with the horses," she said. "He was busy filming it when the tour was going on, so they had some guy replace him -- that was, like, the most depressing part about it ... I was, like, this heartbroken 10-year-old."
Jackson shared her story about Efron while discussing a number of "firsts" with Fallon. She said the first time she was "starstruck" was when she met rocker Alice Cooper after his concert in October.
"I got to meet him backstage. I was like, 'I love you so much. I've always listened to you. I grew up loving you.' I was close to tears," the star admitted. "He was really nice about it. He was like, 'Yeah! Right on, dude.'"
Jackson is the only daughter of late singer Michael Jackson, who died at age 50 in 2009. She opened up about her sheltered childhood with brothers Prince and Blanket in the April issue of Harper's Bazaar.
"Growing up, I was treated as the favorite because I was the only girl. I was the princess; I was perfect in my dad's eyes," the recent Star guest star said.
"The first 12 years of my life I was homeschooled," she shared. "Which means that the only interactions I'd ever had were with family members or other adults."