A friend you kind of know, enough to be besties with them when your real best friend is absent from school or can't go to a party. Someone who usually isn't your first choice, but is nice enough for you to be temporary friends with them until you find your real friend(s). But afterward, you will probably never talk to them again.
Sarah: Are you going to Marcia's party since you guys were hanging out during chemistry class?
Jill: No. She was just my Substitute Friend.