Question - my teacher says that when I appear on Zoom, I show up as "Guest." How do I fix that?

Assuming you're in the zoom application window....
Zoom_and_My_Settings_-_Zoom_and_2020-04-06_10_00_27_Harlan_Howe_s_Personal_Meeting_Room_3421705969.png
Click on the icon in the upper right. (This might be your picture or your initials...) Choose "Sign out."
You'll be sent to a sign in page.
Instead of entering a username and password, choose "Sign in with Google." (This is assuming you're logged into Google for email and so forth already on your computer.) 
Make sure that (if prompted) you are selecting the school account you've been given.