For school computers, we've seen several issues that interfere with App Inventor, including firewalls on school networks, restrictions on software the the machines can run, and ports being blocked. There are many things that can go wrong, such as unexpected configurations, restrictions on the network you are using, or other software running on your computer that interferes with App Inventor. The process that starts the emulator involves several programs working together. The emulator should start on your computer, and connect to App Inventor so you can test your app and do live development. To use the emulator, choose Emulator from the Connect dropdown menu.
App Inventor provides an Android emulator for people who do not have Android devices, or who would prefer not to use one while creating apps.
