Mahouka Koukou no Rettousei: Koritsu-hen  

The Irregular at Magic High: Isolation Arc, 魔法科高校の劣等生 孤立編

The manga version of "Isolation Arc" from the 23rd light novel is called "Mahouka Koukou no Rettousei: Koritsu-hen" (魔法科高校の劣等生 孤立編).