We could create an event that explains there disappearance rather than just having them drop off the face of the planet. Say they're working in the armoury and a batch of dust they were experimenting with ignites violently. They'd all horribly disfigured and as a result sent home to recuperate. Or some similar accident. It would make the most sense to me since I don't think any of the members of RPGD seemed like they were having problems at the school so something that forces them to leave would be better than just having them disappear.