“I believe it is a federal law that you must be 21 to purchase a handgun.”
You are completely wrong. Gun ownership age limits are a state matter.
Not completely wrong. You cannot buy from a gun dealer (fed law) but you can from private party or possess as a gift (Missouri law). Still, how would the coach know? Does he inventory all private property upon joining the team? I think if someone lost a scholarship over this, they would have a case. Does he seek to limit freedom of speech> Waive your right to a trial by jury, post bond, illegal seizure, etc?