Demons have been around almost as long as Angels have. They are the opposite to Angels in every way shape or form, where angels try to help the human race demons do whatever they can to hinder their continued existence. The demons that attend the academy are there entirely of their own choice, some want to try improve their ways whilst others are there to get a handle on their powers.
The power of a demon depends entirely on the type of demon it is. Some are little more than tricksters that get in the way of people's everyday lives whereas others have the ability to completely mess up others lives. Demons powers mainly revolve around messing with the natural order of things however it can appear as something as simple as creating a fire or something as drastic as a curse.