Zac Efron, Vanessa Hudgens, Ashley Tisdale, Corbin Bleu, Monique Coleman and Lucas Grabeel will star in the third and final installment, a feature film. The first two movies aired to a record number of viewers on The Disney Channel.
Director Kenny Ortega tells PEOPLE he's excited about working one last time with the cast, a group of virtually unknown actors before the first HSM movie's January 2006 debut.
"I am the luckiest guy in the world and I know it," Ortega says. "This will be our last time together and I love working with these kids so much. It's going to be great."
Tisdale, who will reprise her role as school diva Sharpay, says she looks forward to returning to East High set in Utah one last time.
"I'm excited," Tisdale tells PEOPLE. "But it's going to be sad. It seems like it will be graduation and I'll be crying while we're filming."
According to the studio, HSM 3: Senior Year finds high school seniors Troy [Efron] and Gabriella [Hudgens] facing the prospect of being separated from one another as they head off in different directions to college. Joined by the rest of the Wildcat crew, they stage an elaborate spring musical reflecting their experiences, hopes and fears about their future.