Interesting discussion.
Of course, things will look pretty different again in about 10 years. New schools will be considered "big" around the Des Moines area. Dallas Center-Grimes and Norwalk will both be over 700 BEDs in 2-3 years. Waukee will split in 4-6 years. Keep an eye on Johnston and SE Polk. Within 5-10 years they will be close to 2000 BEDs and could be pondering split high schools as well. That's easily five new 4A schools in 10 years.
The next wave will probably come from the Bondurants, Ballards and Gilberts of Central Iowa - give them 10-15 years and they could well be knocking on the door of 700 BEDs.
Keep watching population growth. The "suburban circle" of the Des Moines metro continues to expand outward - don't be surprised to see towns like Granger, Perry, Pella, Knoxville, Adel, Carlisle, etc. become hotspots for growth.