In place of regular classes, the students learn more about how to defend themselves against dark wizards. They're learning more advanced magic, as it's necessary to survive.

The common rooms have been magically enlarged to make extra room for those staying in Hogwarts. Dormitories are used as sleeping quarters, and common areas are used for recreation and relaxation. Again, it isn't necessarily divided by houses as it once was, because now it is used by not only students but other adults.

The library is still used for a resource, as well as entertainment. Since people can't get out of Hogwarts very often because of the danger, they tend to pass the time by reading or studying.

The hospital wing is still available for those that need medical attention. It's become quite the necessity nowadays, unfortunately.

The corridors and passageways are still in use, as people need a way to get from room to room. The changing staircases still have a mind of their own, so be careful.

Need to send a letter to someone? There's always a risk an owl could be captured and the letter read, so be careful what you say. Most of the owls make it to their destination, it's just no one ever knows what condition they'll be in.