High School Musical
The first in a triology of films about young kids at school singing. Mark's daughter informed him that he was "not a proper film critic" as he hadn't seen this or its sequel, as both made their debuts on the Disney Channel and were released as DVDs; only the third, High School Musical 3, received an actual theatrical release.
The films inspired the High School Musical Rule - One is great; Two is bad (in this case, too much smooshing); Three brings it back. Other trilogies that follow this rule are Back To The Future, Lord Of The Rings, Ocean's 11 / 12 / 13, the first three Die Hard films, the Transporter films and The Matrix. (Conversely there is the Evil Dead rule, where the sequel is even better than the first, but the third is utterly terrible). That said, in David Morrissey's house, HSM1 is rated as the best of the three.