Continuity mistake: When Yuka Nakagawa is poisoned, her face lands in the bowl of poisoned spaghetti. A little later as the girls begin shooting each other, her face is now out of the bowl, and on the table instead. There was no movement to suggest that her face fell out of the bowl, as it quite clearly happens between shots (just before the girl jumps ono the table.)
Continuity mistake: When Shuya finds Utsumi dead, the proximity of her head to the girl behind her changes between shots, as does the position of her tie and gun beside her.
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.