Try to negotiate with her

"Tiffany I can't have sex with Mark. My mum will find out. She is having me taking a virginity test from time to time. Isn't there something else I could do?"

Tiffany smirks, "How about your anal cherry? I think Mark would love to stick it in there, too."

"No way! The bible forbids anal sex. It is evil and against natural law."

Tiffany rolls her eyes, "So you agree to let Mark stick it in your pussy then? I don't mind either way. I will help you with your mum. We are friends and I know the doc in the local clinic. For a small favour he would attest your virginity any time. Hell he would even do that if you get yourself preggers."