The private Soyogi Academy.

The sons and daughters of the nine noble households- referred to as the "Suuke (Number Families)"- attend this school. Within the school, these students are treated as superiors by their peers.

They have a secret- in this school, there is a war that only they’re able to fight in.

It is called the "Suuransen (Number War)".
Since the olden days, this periodically-fought war has been determining the rankings of the Suuke, as well as selecting a "Soyogi" who leads and controls the Suuke.

This war was thought to never take place again after one number broke away.

However, since the sudden re-emergence of “the number that should not exist”, the conditions for the Suuransen to be held have now been met…

“When the time comes in which the ten numbers assemble,
The carved seal shall guide you to the field of battle.”

Her name is Reisaki Hiroka.
She is the one who signals the beginning and the end.

[Translation of the official website's story summary.]