A half-hour or so, maybe? Probably everyone had gotten to the stadium but then there was the accident and Dixie's Pokémon were injured, so class was dismissed early and everyone dispersed to do whatever they wanted, but then a half-hour later, Brittany made the announcement. I dunno.
It's another new game. I don't know why wouldn't they base an arc on it? That would be a really ugly omission. FR/LG Saga wasn't based on the games too much, aside from taking place at Sevii. They can make a really decent original plot based really, really loosely on HGSS. After all, they have one very important and infinished plot. Pryce.