Blue Exorcist, Vol. 4 by Kazue Kato

Rin and his exorcist classmates are caught in a secret war against the forces of darkness.

Raised by Father Fujimoto, a famous exorcist, Rin Okumura never knew his real father. One day a fateful argument with Father Fujimoto forces Rin to face a terrible truth—the blood of the demon lord Satan runs in Rin’s veins! Rin swears to defeat Satan, but doing that means entering the mysterious True Cross Academy and becoming an exorcist himself.

While most of the students at True Cross Academy head home for the summer break, Rin and his classmates are sent to a training camp in the forest district, and the right to go on a real mission is riding on their performance! Locating and recovering their objectives in the woods is tough and will require teamwork, skill and more than a little magic—and Rin will have to learn to rely on his friends. But is Mephisto masterminding something behind the scenes?!