Chapter 6 of Year 3 in LEGO Harry Potter Years 1-4 for the Nintendo DS sees the player guiding Harry on a chase through Hogwarts to catch up with the elusive Peter Pettigrew. Finding Peter will be complicated by the presence of Dementors, Prefects and other obstacles, but it can be done if you're careful and alert. In particular, always be aware of the location of the Prefects; don't let them see you.
Begin by heading to the right. A Prefect will be moving around this room, so avoid him. The statue in this room transforms into a platform when you use Transfigure on it; climb it, jump from the first bookshelf to the second, get the key on top of the bookshelf and leave the room, always avoiding the Prefect.
Open the locked door using the key. You will have to clear some obstacles, destroying some and moving others. Once you've done so, you'll see a pressure switch. Jump on to it. A wall will move out of the way. Continue down the hall.
Destroy the three Dementors which will emerge from the end of the hall. Doing this will cause a chest to appear. Open it and collect a cauldron. Place the cauldron on the middle of the three nearby pillars and a door will open.
Walk through the door; more Dementors will oppose you here, so destroy them, then destroy the books which are in front of the nearby door. Once you pass through this door, you'll find yourself in a room which features a password-protected door.
Reveal the three paintings in this room to find out the password for the door. Use Wingardium Leviosa on one wardrobe, Transfigure the other one, and destroy the bookcase. Destroying each of these obstacles will reveal a painting; the three paintings combined will give you the password. Head over to the door on the left, which you can now open.
Watch out for the Prefect in this area. Blast your way through the Dementors here. Pick up the painting on the right and move it to the shining point on the wall. When the door opens, head down the hall, downward and to the left. At the end of the hall, another painting will give you a clue and open a door for you.
Enter the newly-opened door and fight the Chameleon Ghouls you find there. When all three are defeated, a chest will appear containing the key that opens the last door in this area. Zap the objects that block the door, then go through it.
Cast Lumos to brighten the new area. You will see that there are two Prefects patrolling this area, so give them a wide berth. Your objective, Peter Pettigrew, is also close by. Head up the hallway on the left, then head right toward Peter's location. You will find three statues; build one, "Transfigure" the second and use "Descendo" on the other. You will find that one is incomplete. You need to locate and restore its head before you can proceed.
Grab the head, which is marked on your map as a blue star. One of the Prefects is nearby, so you'll have to time this carefully and grab the head while he's moving away from you. Return to the statue and restore the head. The door will open.
Chase Peter as he runs away from you. You'll find a closed door with a nearby painting, who asks you to rehang the room's other paintings. There are four paintings in this area. Some need to be repaired, while others have to be levitated into position. One is trapped under a statue and one is hidden inside a crate. Once you have all four paintings in position, a secret door in the same hallway will open.
Enter the secret doorway. Tap the snake head you find and speak Parseltongue. You will now be able to get through the formerly closed door. Once again, you'll see Peter running away from you.
Head down and to the left. Another squad of six Demetors will appear. Defeat them and keep heading in the same direction until you encounter a painting, who will tell you that there is a secret door nearby. Smash the nearby crate and use the pieces to build a statue; the secret door will reveal itself.
Go through the secret panel. Your objective is the marked door, but there are two Prefects in your way. Carefully shadow the Prefect who walks to the right, staying behind him but at a safe distance. Eventually you should be able to make a dash for the door.
Use Lumos to illuminate the new area you find yourself in. Peter will flee again; chase him to reach the end of this level.
There are many other secrets hidden throughout the level, including Wizard Hats and cards. Experiment with your surroundings to reveal them.