From the description, it sounds like the school was built on the edge of the town and is basically up against farmland on the north and another side.
Either way, if the hunters/pasture being hunted was 600 yards away, then 600x3=1200 which still puts any hunter outside the GFSC of 1000 feet.
Ballistically, most deer rifles will shoot further than the 600 yards, so knowing your backstop is necessary when shooting in most midwestern farmlands ... very few hills/wooded areas to help stop rounds.
Now, according to what was written as far as the LEO investigation, I would not consider the hunters to be negligent, but rather the land owner for not notifying them of the recent change in the addition of the school and where it was located and how they should adjust their shooting lane. If these were lease hunters ... basically, hunters from another area, and not local residents who should themselves have known about the school, etc. And, if the land owner did inform the lease hunters about the changes, then they will have some responsibility about what happened.