Blue Exorcist, Vol. 4 : 4

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. Can Rin fight demons and keep his infernal bloodline a secret? It won't be easy, especially when drawing his father's sword releases the demonic power within him!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?!