The Final Order - The Natural Order School of Magic, #4

The end is near.After narrowly escaping Ilana's dangerous school, Tristan is reunited with his friends once again. But though Ilana is gone, the deadly enchantment she left behind is powerful enough to bring the world to ruin. Time is running short as Tristan and his friends fight to contain the destructive magic. And this time, there's nowhere to hide.