Here are the instructions
http://appinventor.mit.edu/explore/ai2/setup-emulator.html , be aware there you also need to use the Mac part of the instructions.
You are running this on a school network. It is very possible the network is blocking the install. You may have to get the help of the school IT people to install this. If you are in a class using AI2, this should already have been done. If you are an instructor trying to set up App Inventor without the help of your IT department, the firewall might be blocking ports 8001 and 8004 . App Inventor needs to use those ports.
Regards,
Steve