Note that if you want to use the USB cable instead of the Wifi option, you can still use the online version. You just need to install aiStarter on the school computers and use the USB option from the Connect menu (the last two items from Anke's list).
As Steve mentions, only the online version is authorized by MIT. Any offline distributions are not provided by us (App Inventor is open source, so anyone can compile a copy). If you do go down the offline route, you'll want to make sure you are backing up project data as well.