I figured it out, a lot of times they get stuck on the faaaaaaar right side of the high school, or seldomly in the ceiling, I just blast em with the plasma cannon if that happens. They tend to come out in batches too, and until they are all dead, no more come out.
Staying in the pool, and running with the spear from group to group (while tagging 3) is an easy way to kill them quickly, and get more points.
A regular kill is 100, a tagged kill (going into first person and hitting x when they are in the crosshairs) is worth 250.
I have no idea, because of that - how many I killed, but it was a shiiiiiiiiiiiiiiiiit ton!