Honestly, that could be it. I remember when I was in 11th grade, this new kid at my school died in a car accident. He was in one of my classes, yet no one knew who he was. It was pretty sad how my teacher had to try to describe him to us, and even then, most of the class had trouble remembering him, despite the fact that he had been in the classroom every day for months prior. In every class, we had kids who were very quiet, never said a word, etc. If Abigail was similar to that, I can see why her disappearance is not a big "issue" to her classmates. Also, if the kids have convinced themselves that she ran away or she met someone on the Internet (vs. being snatched by some predator), I could see why they wouldn't be too concerned. Maybe a lot of them don't truly get what happens when someone goes missing. Has the school issued any warnings? Did they have an assembly? Send a note home? New security procedures? If the school or town isn't treating it like a stranger abduction, then it's not surprising the students aren't reacting to it like one.
ETA: Did Abigail go to middle school with her 9th grade classmates?
