The fact that half the student body might have known the entry code to this house is not in and of itself a clue as to who followed either the girls or the couple home with the intention of killing them, at a time when only the legitimate residents of the house were inside and presumably had the doors locked. But it may be related. We won’t know unless they identify and arrest a suspect. There are a lot of those cars around and then there’s the assumption that the killer was in that car, which may not be correct, or that the car was owned by the killer or somebody known to him, again not proven.
I'd go down the list of who passed off the code to whom. People would be more than willing to help in a situation like this. Btw, did they say if the door entry code was disarmed? Of if an accomplice was suspected?