This is how to get in ... (highlight below)

Drag the rock hidden behind the plant over to the mirror and let it go. The mirror will break.
Click the key and put it in the keyhole.
Click on the note behind the mirror and type "reparo" to fix the mirror.
Then click the door at the end of the hall in the mirror. That will open the door.

Then, click on the tassel on the graduation hat and drag it straight up. The test will appear. You have 20 minutes.