Emily:
Debut -- The Puppet Show (Season 1)
Died Same Episode
Emily appears in the first few minutes of The Puppet Show as one of the students who's in the talent show. She's a dancer, and we see her stretching out in the locker room. Without any warning, we see something holding some type of a knife and we hear it saying something like, "I will be flesh!" Basically, Emily had no lines in this episode, and although we hear from other students that she was a really good dancer -- actually, Mark is the one who says this -- we never really see her do much of anything. Someone finds her in the locker room with her heart removed, and Buffy and Co. are left wondering what exactly happened.
Eric:
Debut -- Some Assembly Required (Season 2)
Last Seen -- Same Episode
Eric is a friend of Chris's...he's seen taking photos of girls and we hear him and Chris talking about something suspicious. Turns out they're trying to make a girl and they're collecting body parts of girls who have been killed in accidents. Eric believes that since they're so close to finishing their girl -- who would be the girlfriend of Chris's older brother Daryl, who's disfigured as the result of an accident -- they should just go ahead and kill someone for the rest of the parts that they need. This wasn't a favorite episode of mine, actually. We don't know what becomes of Eric -- whether they've put him in a juvenile delinquent hall or what. But that's the last we've seen of him.