To find your school code, click on "My Account" at the top of the EEi screen after logging into your teacher account.
In the first column you'll see your School Code. You can change this to whatever your like, as long as it is unique in the EEi system.
Here are some quick tutorials on the School Code and getting your students into EEi.
https://www.eeiblog.com/eei-school-code
https://www.eeiblog.com/school-account