My suspicion is that Cedric is under a Deathly Hallow invisibility cloak, instructed to leave and send a Patronus to Dumbledore at the first sign of serious trouble, and furthermore Memory Charmed Harry to forget him so that no one could read Harry's mind and figure out that backup plan.

I also suspect that the next chapter will contain a long discussion between Harry and Quirrel before any serious action, but Quirrel might want to have that long discussion after they're in the final chamber, not before.