What is best course of action to protect myself as a high school coach when false sexual allegations are threatened in retaliation for a player's removal?

The best thing you can do to protect yourself is to get as many notarized affidavits as possible from credible witnesses who were in a position to observe for themselves whether the alleged conduct took place. Statements would also be useful from anyone who would be in a position to testify directly that the player in question never complained of the alleged behavior. It's also a good idea to let it be known that you are prepared to defend your reputation to fullest extent of the law and that any false allegations will be met with a lawsuit for slander (if the allegations are made orally) and libel (if the allegations are in writing).  It's not a bad idea to do a little Internet research for cases reported in the media about just how much legal trouble people can get into when police charge them with making false statements.  Let it be known that you are prepared to cooperate fully with the authorities in any subsequent criminal prosecution.