Which school? I had Georgetown. They explicitly said that to call the admin at your interview time (they will accept your friend request at that time). You will be deleted from their Skype after the interview ( I guess they don't want to keep all interviewee in their contact list)
Hello there,
If you are nervous about things, you can always write the school or interviewer an email saying that you have added them as a contact. That way if anything goes wrong (like technical issues with Skype) at least they will know that you did what you were supposed to. When the interview time should be, you can always write them a text message in Skype with the word “Hello” or “good morning”