I originally interpreted the following as subtly implying the students were using prediction markets

A sudden air of attentiveness, as of long-standing disputes about to be settled. "Well, finally," someone said, as Millicent tried to catch her breath. "He's only got, what, ten days left to go bad?"

"Eleven days," said the seventh-year who was running the betting pool.

But it seems more likely that eleven days is how long the year has left.