Disney Channel (at least in Australia) once had a tendency to overpromote any of the films if it's set to air within a week. Almost every single commercial will have at least one ad about it. Considering that, at least between 2005 - 2010, the film aired once a month, this was quite common.
Since 2015, Disney's main movie channel in Australia tends to play the third film every week, and will commonly play songs from the three films between movies. Descendants pretty much took the trilogy's place on Disney Channel itself.
The Humuhumunukunukuapuaa song and scene were cut from the televised version of High School Musical 2. They reinstated it on the DVD and in the stage adaptation.
There was also a song recorded for the 3rd movie called "Last Chance" that never made it into the movie apart from the reprise in the medley of most of the songs in the movie. A 40 second preview leaked in 2008 before the movie came out.
Many of the actors were actually in their early 20s, though when the films initially started, most of the characters were played by people close to their actual age. Taylor's actress, Monique Coleman, was 26 at the time of the first film.
Averted by Austin Butler in the spin-off film, who was nineteen and playing a college freshman. Likewise Bradley Steven Perry as Sharpay's rival.
Development Hell: A fourth movie was announced back in 2016 and casting had begun. We haven't heard anything about it since then.
Divorced Installment: The film originated as a third Grease film, where Britney Spears and Justin Timberlake would have played the children of Sandy and Danny - with the Sharpay-type character being Rizzo's daughter (played by Christina Aguilera).
Dyeing for Your Art: Zac Efron had to practice basketball three hours a day during the first movie.
Fake American: Oleysa Rulin (Kelsi) is a marginal example, as she's Russian born but was raised in America.
Follow the Leader: High School Musical's success was a clear factor in the format of Glee - to the point that in its early days, people thought it was a Deconstruction of HSM. Camp Rock was also clearly inspired by it. Nickelodeon also produced the similar Spectacular.
Genre Relaunch: Disney's first successful live-action movie musical, discounting productions of old musicals, since Pete's Dragon (1977) killed the genre nearly thirty years prior.
Hostility on the Set: It was rumoured that Ashley Tisdale and Lucas Grabeel didn't get along during the first movie.
I Want You to Meet an Old Friend of Mine: Prior to the first film, Olesya Rulin (Kelsi) and Lucas Grabeel (Ryan) had been in Halloweentown High together, as Natalie the troll and Ethan the warlock, respectively.
Incestuous Casting: In-Universe. One character remarks that if Sharpay could find a way to play both Romeo and Juliet, Ryan would be out of a job, suggesting that the two have played romantic partners in the past.
Non-Singing Voice: Zac Efron's singing in the first movie is done by Drew Seeley. This is because Troy was envisioned as a Tenor Boy, and Zac is a baritone range. Troy's vocal range is adjusted in the sequels as a result.
The Other Marty: Cassie Steele was cast as Gabriella based on her performance in Full Court Miracle, but she had to drop out due to commitments with Degrassi: The Next Generation.
Playing Against Type: Oddly, Ashley Tisdale as Sharpay - her previous Disney role was Maddie in The Suite Life of Zack and Cody, a completelydifferent role. If you want to go into detail, Ashley basically played a bitchy version of London in these movies. Lampshaded in an episode of the sitcom where Maddie tries out for Sharpay in her school's production of it, and gets told she's too nice for the role.
Romance on the Set: Between Zac Efron and Vanessa Hudgens, who were together for over five years. They confessed to liking each other during the first film and got together during the second. (Although they hid it for as long as they could to avoid media attention). Kenny Ortega admitted that he'd noticed their chemistry since the beginning, and deliberately paired them during auditions.
Sleeper Hit: Seriously, nobody thought this Disney Channel Original Movie would end up being a giant hit that would encapsulate the late 2000s.
Star-Making Role: For Zac Efron & Vanessa Hudgens. It's hard to believe that Ashley Tisdale was a more recognisable name than them at the time.