It took four days to film all the school scenes at East High. During one of those days, classes were in session. Actual students at East High were given instruction of where not to go so that no one would interfere with the filming of the movie.
Drew in a rating of 17.6 million viewers on its premiere night, making it not only the highest Disney Channel Original Movie in history, but the highest in cable history, even higher than NFL Monday Night Football (1970).
The sheets of paper thrown in the air during the "What Time Is It" musical sequence are actual assignments of real students from East High. The papers were all individually checked to make sure there were no embarrassing pieces of information on them.
Before filming this movie, Monique Coleman did not know how to swim and was actually afraid of the water. For the scene where her character jumps into the pool, the script was rewritten for her dancing partner Corbin Bleu to jump in with her. She later said in an interview that she received an amazing amount of support from the entire cast and crew.
Any time an actor hit a golf ball towards a camera, they were actually hitting table tennis (ping-pong) balls. This is clearly evident during the Bet On It scene as the ball Troy goes to hit on the Tee has no grooves. In this shot Zac Efron hit several balls and all towards cameras manned by crew members and people around them. If real balls were used they could have not only hurt the crew, but also to not break the cameras
The coaster of people doing the wave, at the opening scene in "What time is it" was started by the HSM cast/dancers and Kenny Ortega. Since all the cast/dancers were sore from dance rehearsals, they sat in a line on the floor and began to massage each-other. When Kenny saw this, he told them lean to the right (one by one in a wave), and so they did. Then he decided for them to try it standing up, and when they did so, he used it in the movie in during the opening song.
During the final chorus in "I don't dance", the dancers and players are moving and singing faster because it was getting dark and they needed to finish quickly. So during the lunch scene, it turned dark and a stage light was used to give the illusion of sunset.
There are pictures of Sharpay's and Ryan's "Bop To The Top" performance from High School Musical (2006) in Miss Darbus' room, and pinned to Sharpay's dressing room mirror. There is also a "Twinkle Town" poster in Miss Darbus' room.
"Everyday" (performed by Troy (Zac Efron) and Gabriella (Vanessa Hudgens)) is the only duet between the two in the High School Musical trilogy that is performed in full costume. In High School Musical (2006) Troy and Gabriella were auditioning when they sang, and they wearing their school clothes. High School Musical 3: Senior Year (2008), Troy and Gabriella drove from Stanford to be at the performance and sang in what they were wearing when they arrived.
At the start of a dinner scene at the country club, sharpay's father is mid way through a story of which only the line "...and she stepped on the ball!" is heard. This exact part line is also used in the film trading places in the same way, at dinner in a country club.
Zac Efron who played Troy Bolton in HSM1(2006) soon after HSM1 dyed his hair and switched from a natural blond to brunet in HSM2 & HSM3 without anyone of the cast noticing it.The script offers no such explanation.Zac Efron is still a brunet in real life.
Charles Klapow: The main choreographer can be seen in almost every dance sequence with a long, dark mohawk. In "What Time Is It?", he is in the basketball dance, and he does a handshake with Troy before he sings, and is wearing a red short-sleeved shirt over a white long-sleeved shirt. In "Work This Out" he is in a green Lava Springs shirt and work pants, in the middle of the three men with pepper grinders, during "Everyday" he is wearing a silver vest over a pink shirt who dances across the line crossing paths with Martha, and in "All For One" he is wearing a blue deigo tee with red trim, seen dancing with a girl in yellow during the choreography, and with Miley Cyrus.
The trivia item below may give away important plot points.
At one point in the film, Sharpay refers to the Wildcats as the cast of Grease (1978). The original High School Musical (2006) was created by editing the unused script for Grease 3. Additionally, Vanessa Hudgens (Gabriella) would later go on to play Rizzo in Grease Live (2016)