This is another simple one, and easy to do. Go into the gym area of the school, and turn on jetpack mode. You must go to the actual gym, and fly up as high as you can. You will see the two hallways, fly into one of them. When you land, you will find the rest of the level missing. If you go back into the gym, you can fix this bug.
Click on a thumbnail image above to view a larger sized image.