Population may drop by that much, but I don't know if that will lead to an overall reduction in schools. A lot of smaller schools are already as consolidated as they're going to get. I could see financial pressures causing a consolidation like Parke Heritage's in some places where you might have two schools in shrinking communities close to each other like you might find in Wayne or Randolph Counties, but I don't see too many of them. What it *may* do, especially without co-ops, is shrink the number of schools playing 11-man football.
A much bigger issue is if this hypothetical goes through (and it likely won't) is that Illinois allows co-ops - and had a long, long-standing tradition of co-op programs (especially in football). Indiana does not. That could lead to some issues.