Click Add to save. It is helpful to name the class something like "demonstration" or "demo" to easily distinguish it from the other classes in the school.
It is helpful to use "Demonstration" and "Student" as the first and last name for the new account, to make clear to others accessing school data this is not a real student account. For example, first and last names could be Demo Student1, Demo Student 2, etc. The usernames could demo1, demo2, etc.
After your staff is done using the demonstration accounts, you should delete accounts so this data is not integrated into the overall school data. Click the check box next to the student's name and then click delete students. Or you can select the demonstration class in the Class filter and click the checkbox at the top of the list to select all students. Then click Delete Students.