On the front steps of East High, it's the first day after winter break ("Wildcat Cheer"). The Jocks, Brainiacs and Thespians find their cliques, recount their vacations and look forward to the new year ("Start of Something New"). In Ms. Darbus' homeroom, basketball team captain, Troy, discovers that Gabriella, a girl he met singing karaoke on his ski trip, has just enrolled at East High. When he calls her, the eccentric drama teacher quickly confiscates all cell phones and assigns detention. Between classes, Gabriella and Troy look at the audition sign-up for the school musical, but the drama diva, Sharpay, discourages them. At basketball practice, Troy tries to shake his desire to sing at the urging of his best friend, Chad ("Get'cha Head in the Game"). Taylor, the science club president, discovers Gabriella's intelligence and encourages her to join the upcoming science decathlon.

While the students learn how to "act" at detention, Troy's dad, Coach Bolton, faces off with Ms. Darbus – his star players can't miss practice because Friday is the championship game! The next day, hopeful Thespians strut their stuff for Ms. Darbus ("Auditions"), but they're no match for Sharpay and her twin brother, Ryan ("What I've Been Looking For"). Troy and Gabriella arrive too late to audition, but Kelsi, the show's composer, plays the song her way and encourages them to sing ("What I've Been Looking For – Reprise"). Ms. Darbus overhears and gives them a callback, which turns the school order upside down! Sharpay is furious, and some students try to break out of their cliques during lunch ("Stick to the Status Quo"). The Jocks and Brainiacs devise a plan to trick Troy and Gabriella into forgetting about the musical and committing to their respective competitions ("Counting on You"). Thinking each one has betrayed the other, Gabriella and Troy are crushed.

Against Ryan's advice, Sharpay convinces Ms. Darbus to move the callbacks to conflict with the science decathlon and championship game. Chad and Taylor put their heads together to make things right again ("We're All in This Together"). While the Brainiacs and Jocks compete, Sharpay and Ryan pull off a polished callback performance ("Bop to the Top"). When Taylor's laptop shuts down the electricity, Troy and Gabriella rush to the theatre, but are too late for their callback. When the East High students arrive to rally behind them, Ms. Darbus relents, and they sing their way into the lead roles ("Breaking Free"). Back at the gym, the Wildcats win the game, and the whole school comes together as winners ("We're All in This Together – Reprise," "High School Musical Megamix – Bows").