In reply to mgco3:
My theory is that Mallory may have reached the summit alone:
Mallory and Irvine reached the base of the second step together (possibly confirmed by Odell's sighting of them), this would have presented a formidable obstacle - Mallory as the more gifted climber attempted to scale this and eventually succeeded by employing combined tactics (reaching past at least some of the harder moves by standing on Irvine's shoulders, the rock face is only around 16 feet high), but this left Irvine unable to follow - apparently this is also how the Chinese expedition of 1960 claimed to surmount the step.
They were under the impression that the summit was close so Irvine waited below the step for Mallory to summit and return - the might explain the rumour that the Chinese expedition (the next men to reach this point) found a body on the ridge (unlikely to have been Mallory after the discovery of his body elsewhere in 1999).
Mallory was known for his forgetfulness and combined with operating at extreme altitude he fatefully forgot to take the camera from Irvine to the summit - so in this version of events if the camera were ever found on Irvine it would not have a summit shot.
Mallory continued alone and from the top of the second step the path to the summit is clear of major obstacles, so I believe he may have made it. The distance was greater than he or Irvine anticipated though and Irvine may have strayed from the point where they separated to seek better shelter as he awaited Mallory's return. As an exhausted Mallory (by now operating without supplementary oxygen) gets back to the second step darkness is probably falling, and after arranging a makeshift abseil descent it may well have been fully dark - in the dark and with Irvine possibly nearby but unresponsive after hours alone in the cold, Mallory concludes that Irvine must have headed back towards their high camp and elects to continue his descent alone. Before he reaches their camp he falls, ending up on the slopes where his body was discovered in 1999. The fact that Mallory's snow goggles were found in his pocket supports the theory that he was still moving after dark had fallen. The lack of the photo of his wife that he reputedly intended to place on the summit could also be a pointer that he reached the top.
What happened to Irvine's body (if it was ever seen by the Chinese) is a mystery, but perhaps it was relocated by storms or continues to lie hidden.
...well it's a theory and has less flaws than some others I have read.
Definitely read "Ghosts of Everest" along with Hoyland's "Last Hours on Everest" it is essential material for any investigation into this mystery.
Good luck, although it will be hard to write anything on the matter that has not already been said. For my sins I am also an EPQ supervisor at the College I work in, so would be fascinated to know how you get on ...are you finishing your project this academic year or next?