In order for a Student to have a PIN, they must be added to the family as a contact.
First, create a new drop-down for Contact Type to identify the Student Contact by going to Tools > Edit Settings > Drop-down Lists > Contact Types. Once the new drop-down value is created, find the family and click the Add Contact button.
See our Help article on Contact PINs for more information on how to work with PINs for a Contact.