This Is the first book on the series Scythe.

A world with no hunger, no disease, no war, no misery: Humanity has conquered all those things and has even conquered death. Now Scythes are the only ones who can end life, and they are commanded to do so in order to keep the size of the population under control. Citra and Rowan are chosen to apprentice to a scythe, a role that neither wants. These teens must master the "art" of taking life, knowing that the consequence of failure could mean losing their own.