This book is the third book of the „To the Rescue“ series or the second part of the essential story. DrT writes:
The Sextet is given their first preparations for war, Voldemort seeks allies, Draco makes two errors of judgement, and one member of the Sextet is lost.
Several students are killed. Others are expelled. This book covers the time of the start of school until the beginning of the yule break.
It is rated PG-13.
The Story can be viewed online at Schnoogle.
I made a PDF of its 10 chapters.