I am helping coach football at my former high school (hence the name, "Offensive-Liner") and would like to be able to listen in on our co-ordinators play calls without needing the school to buy another package of headsets.
Good luck. Most systems use spread spectrum as mentioned above, and many use digital formats as well now. They make these headsets difficult to listen in on for a reason.
And a lot of the high-end systems are also encrypted, but that's mostly in the NFL. 2.4 gHz is a smart move since, even if it's analog, virtually no casual listener can monitor it, and more so if it's SS.