Good topic. I too have been coming up dry trying to find football coaches at games in Champaign and Normal in the last few years. Seems like for every ten teams, only one or two are still using conventional analog (VHF/UHF) freqs. Five to ten years ago I had more success finding teams using analog UHF channels. The majority of high schools must be using PortaPhone or Telex type systems these days.
Still though, I’d suggest scanning through the common itinerant VHF and UHF freqs freqs and using CloseCall/Signal Stalker if you have that capability. (However, usually the only thing CloseCall hits on for me at high school football games is the 450/455 MHz remote radio station transmitters.) I also search between 151-160 and 451-470.
Here is an interesting article about high school coaches' headsets: link
You never know: some high schools or even smaller college programs may still be using simplex analog UHF freqs, but they are becoming fewer each season. Good luck.