Trivia: In the film, the two transfer students are Kiriyama, the silent killer, and Kawada, the winner of a previous Battle Royale. In the original novel, only Kawada was the transfer student; Kiriyama was in the same class selected for this game of death. He was the main antagonist.
Trivia: The Japanese DVD features outtakes from the film. One of these shows that in one scene where a cooking pot is thrown at a girl, the pot actually did hit the actress and hurt her (although not badly). The shot where this happened does appear in the movie, but any sign of her having been hurt was edited out.
Trivia: In the bungalow, two female students, Hirono and Mitsuko, are having a beef. Hirono taunts Mitsuko, who loses patience and cries out, 'I've had it! Why does everyone gang up on me, what did I do!' In a website dedicated to the film, it states background information [most likely from the manga] that Mitsuko is the 'class slut', hence Hirono's accusations of Mitsuko stealing her boyfriend, among others, is justified.
Trivia: In the middle and near the end of the film, Kitano is seen and heard conversing with his daughter over the phone; that voice belongs to Maeda Ai, actual older sister of Maeda Aki, who plays Nanahara's girlfriend Noriko.
Trivia: In the manga, Utsumi Yukie is described as the class president, hence her reputation of the leader is obvious in the lighthouse scene. Lead character Nanahara Shuya, as the deleted scenes show, is the star basketball athlete, hence his popularity.
Chosen answer: Kawada hacked the system, learned about the collars, and transferred voluntarily to the class he knew would be participtaing in both versions. The only difference is when he transferred: in the book, it was right after his win, and in the film it was just for the battle. Shuya never hacked anything in either the book or the film. As to why the change was made, I can only assume that, given the shortening of novels involved in film making, it's easier to make Kawada a complete stranger than a loner that the kids recognize.