This information gets back to the 16 year olds girlfriend who calls you wanting answers. You tell her that you did not fuck her boyfriend & never would, so please fuck off. Drama ensures.
Ignore it all
Tell 16 year old to take care of it himself
Tell them all to go fuck themselves, you're 21 & would never touch a 16 year old. If they want to keep going they'll end up with a fist in their faces
Other (please explain)