Sending a fake or spoofed e-mail is never an appropriate action. This is purely for educational purposes.
Have an example of a spoofed e-mail available for your students to look at. It can be similar to the following:
From: Your Boss Reply To: Jane.Smith@email.com Subject: You're due for a raise! Body: Please come see me at a later point today!
Start a discussion about this e-mail. Questions can include: