Any time you are completing a registration that is not solely for yourself, please use the "Register a Group" button. Registration Option Dialog Box

After selecting "Register a Group," the system will show you all contacts that are associated with your school. Additionally, it will show whether a particular contact has already been registered for the event, regardless of who completed the registration. 

already registered example

If the person you wish to register is not listed, you can add them really easily by entering their email address and name. Add contact through registration