To get to the bonus levels, you have to go through the initial stages of the "school" experience. Then when you can return to the alley, go to the bank again down at the end. You'll instantly get teleported to the vaults. Each vault down here is a bonus level. You unlock them by finishing off the various regular levels.

The first bonus level is the very first vault on the left. Your task here is to get 100,000 coins. They've made this very easy! In past games you had to get EVERY SINGLE COIN on the level in order to complete it. It was mind numbingly hard sometimes! In this one there are FAR more than 100,000 coins on the level so you don't even have to get half of them. You just have to have some fun smashing things and eventually you'll get there. Be sure to kick the soccer ball into the goals a few times!

When you do, you get a gold brick in the top center area of the level. As soon as you get the brick, the level is over.

Lego Harry Potter Walkthrough