Of course your first task is to gather up tons of coins and kill all the plants :) Feed poison to the venus fly trap to kill it. When you're sure you're ready, go in to the actual class.

Yup - destroy everything in the room. When you're sure you have, build a crate in the bottom left. This becomes a train and breaks into the upper area to free Ron. Make sure you really have destroyed everyting else! This upgrades your Lumos light spell into a more powerful version.

When you're sure you're ready, go into the middle field area. It's broom flying time! First shoot the gargoyle. Then chase around the moving one. I realize it's a pain. Be patient. Practice. You're going to need to fly a lot later on, so just think of it as a task you will get better at as you go. Shoot the flowers on the big plant thing. Gather up more coins.

You can't dig yet, so ignore those prompts.

When you're sure you're set, follow the ghost through the hallway and through the plant wall. Now you're in a new stone courtyard area.

Shoot the plants in the bottom right for one component. Make the gyroscope in the center. The last piece is in the grass in the top right. Pull! Now you can go right.

You're outside! Free! Clear the entire area. There's a boy in the bottom left with a plant - free him for a Student In Peril.

When you've got everything you could, move on down to the Quiddich level.

Lego Harry Potter Walkthrough