After getting the diffendo spell book look for a gnomes. In the same direction will be a rope that you target with the diffendo spell to lower the bridge.
Extra Chocolate Frogs With Voldemort
If you have used the three frogs in the battle before Voldemort (Gytrash), do not worry. You can get these again. Save the game in a safe area in the middle of the battle. Then, exit the game and reload it. The frogs will be there again.
Gargoyle Statues
There are mean looking statues of gargoyles all over the teachers' quests. Light Lumos in front of them and they will reveal a hidden path.
Hidden Items
Search all of the desks in the classrooms. They contain Beans, Cauldron Cakes, Pumpkin Pasties, Stink Pellets and even Wizard Cards.
Invisibility Cloak
This cloak is not too helpful. Prefects will still catch you. The most you can do is not touch the Prefects. Filch can usually find you, even if you do not touch him. If you do get caught, even without the cloak, you will not get House Points taken away from you.
Quest Warning
If you are doing a small quest, like following Hermione or Ron, follow them. Do not take any shortcuts. At night there are not any Prefects outside the common room.
Secret Passage To The 4th Floor
Go out of the common room and into the main room. Turn right and crawl under the bookshelf. This area is also in the Chamber of Secrets.