App Inventor 2 is a great product. But the emulator that is built into the product is not as good. An independent company, BlueStacks, provides a better alternative.
This video describes the process by which you can install the BlueStacks Android emulator on your home PC (or your PC at school if your instructor instructs you to do so). Once installed, you can test the apps you build with App Inventor 2 on your emulator before you download the final version to your phone or tablet.
YOU SHOULD MAXIMIZE THE VIDEO TO FULL-SCREEN SIZE BEFORE PLAYING.