Er, this is not very convincing, even if you phrase your opening sentence "the vast majority of kids who were mugged." It has to be 100 percent, and in this instance it has to be 100 percent of youngsters who watched someone apparently being murdered on stage in a dance club but who don't mention this to any authority figure whatsoever. But anyway, in Episode One a dead body is found in a school locker, and not only is there no cop presence, and no investigation, this leads to no change in the school routine.

My provisional theory is, "No time/room in script to provide explanation, and Whedon et al. are hoping that viewers won't be anal or literal or trivial enough to give a shit about the missing cops."