I've heard the screaming cry with a game that doesn't have enough save check points. We've had to say that game isn't allowed on days when you might not have time to reach a check-point. Minecraft, though, is easy to save and go, but we do have to give the 15 & 5 minute warnings, with verbal confirmation that they look at the clock and acknowledge the time. It's so easy to get lost in a building project.