The giant crowd in this epic commercial are singing 小さな恋のうた (chisana koi no uta, A Short Love Song), which I'm pretty sure the band MONGOL800 came up with. Here's a link to their version.
The catchphrase that gets posted over the CM is 歌え、10代 (utae, jyuudai, or sing it, teens!).
You can bet I sang this song during last night's karaoke session.