Schools are places of safety and learning. They are a harbor for young minds away from the troubling world. However, in Harry Potter 6 we watch Hogwarts turn upside down and inside out with tragedy, worry, and danger. A sign of the times, Harry and his friends must learn to adjust to a world in which there is no safe place. Voldemort is gaining so much momentum and force that even Dumbledore's enchantments cannot protect the Hogwarts community.
Interestingly, we come to understand through Book 6 just how powerful Hogwarts is. It is so powerful that Voldemort twice tries to become a teacher there. We all know that knowledge is power, and Hogwarts is a vessel of hundreds and hundreds of years of study, discovery, exploration, and fine-tuning. Some of the best wizards and witches of the world have attended Hogwarts. Voldemort would have been all too pleased to advance his own campaign from the comfy chair of a Hogwarts classroom.