It's really strange. I've tried resetSim, I've tried forming a group with the stalled Sim and having them watch TV or stargaze with the active Sim, calling them to a meal, etc. I even tried traveling with the Sim to another lot and then leaving them there. The only thing that's worked is going into the gameplay options and clicking Restore Defaults and Apply, and that only worked once. I can still play through the challenge, because the stuck Sim does consistently leave for class on time, and eats enough not to die. It's just weird.