OK! Exclusive: New HSM3 Cast Members Revealed!

(IDA MAE ASTUTE / © ABC )

The hallways of East High School are about to get a little more crowded!

OK! can exclusively reveal two new stars who are joining the forthcoming High School Musical 3.

Up-and-coming actors Matt Prokop and Jemma McKenzie Brown will do their best song-and-dance alongside Zac Efron and Vanessa Hugdens, a source close to HSM3 tells OK!.

Though little is known about their roles, the source adds that the British-born Jemma seemingly scored the role of a lifetime, landing the part over hundreds of other young starlets.

"She’s 13 years old and beat out 600 girls from three countries," the source says.

This will be Jemma's second acting gig, following six appearances in the British series The Amazing Mrs. Pritchard in 2006.

Matt, 17, has Hannah Montana and The Office guest-starring credits to his name. On his official site, the young actor even teases fans by saying that he has a "HUGE announcement coming... that will blow you away!"

High School Musical 3 starts production in April and is scheduled for an Oct. 24 release.

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