How does the Sixth work?

[geebengrrl.livejournal.com]

This has been puzzling me (ignorant young colonial with no clue about how the British school system works that I am).

In Autumn Term, Karen is in the Sixth. Rowan is Upper Fifth, as is Lois Sanger. Jan Scott is in the Sixth also. In End of Term / Cricket Term, Lois is Games Captain, which presumably means she is in the Sixth; and she leaves at the end of Cricket Term, as does Jan Scott.

However, Rowan says that Jan Scott was always a year ahead of her.

So is this just an inconsistency? Or is the Sixth actually two years long and members of the Sixth can leave at the end of either year? Or what?