Some of your classmates might have more than one email address. If so, then the relation between your classmates and their email addresses is not a function.
They can also have more than one phone number, so the relation between them and their phone numbers might not be a function.
But nobody should have more than one social security number, so the relation between your classmates and their social security numbers should be a function. If it is not, then there is some fraud going-on.
Answers & Comments
Answer:
Some of your classmates might have more than one email address. If so, then the relation between your classmates and their email addresses is not a function.
They can also have more than one phone number, so the relation between them and their phone numbers might not be a function.
But nobody should have more than one social security number, so the relation between your classmates and their social security numbers should be a function. If it is not, then there is some fraud going-on.