spreading rumour that she gave him head ...oral?

my friend's boyfriend...ex now but anyways spread a rumour that she gave him head last weekend which is not true like the whole school knows about it and it's not true and he keeps denying it but his friend said he said it so idk we told the principle but its hard for the WHOLE school to know the REAL DEAL
how to get ppl to realize its not true is my question plz no rude answers