ZAC EFFRON, Vanessa Hudgens and Ashley Tisdale have all officially signed up for HIGH SCHOOL MUSICAL 3: SENIOR YEAR. Six of the original stars of the Disney movie have all completed contracts to appear in the third installment in the film series. Corbin Bleu, Monique Coleman and Lucas Grabeel will also revive their roles for the forthcoming TV film which will see the characters facing the daunting prospect of being split up as they graduate to different colleges. Director Kenny Ortega is delighted to see the cast reunited for the final picture. He tells People.com, "I am the luckiest guy in the world and I know it. This will be our last time together and I love working with these kids so much. It's going to be great." Tisdale, who plays school diva Sharpay in the hit franchise, is equally pleased about the prospect of working on a third project - but admits it will be hard to eventually say goodbye to HSM. She says, "I'm excited. But it's going to be sad. It seems like it will be graduation and I'll be crying while we're filming." High School Musical - a made for TV movie - enjoyed phenomenal worldwide success since its released in 2006, turning it into a global brand. Over 7.7 million tuned in to its first broadcast in the U.S. alone and the highly anticipated sequel, High School Musical 2, attracted a record-breaking 17.2 million viewers in America in 2007 - making it the most successful cable broadcast ever. The third installment is due to start filming early this year (08).