15/16 year old teens have sex. They have always had sex. This is not new...just in previous generations they got married at 15/16 to have the sex
If you have a child there is a good chance that child will have sex at that age.
There is no way to guarantee your child will not have sex before they are ready(and some are ready at 15/16..ie very responsible, and emotionally able to handle it)
The best you can do is constantly be aware of t his and constantly talk about the ramifications of this. You also need to make sure your children get lots of physical affection from the parents. I think a lot of time children have sex it's more about the need to be held than the sexual urge. This is true for adults as well I guess.
To the OP...I do not allow my 13 year old daughter to be anywhere that a parent isn't present and on the same page as me. If my gut tells me the parents aren't what they say they are then I have the kids here and I supervise.
I have learned from experience that 13+ year old kids need more supervision than younger children. I learned that when my 13 year old dd and her friends got drunk(and they were supervised that night..the parent was just so unsuspecting she didn't notice..someone saw them and called)
It's ok to tighten the rules when they are teens. It's ok to have allowed her to be unattended at home when she is younger but then when she proves herself no longer trustworthy to change that.
What if she bends the one person rule only to a boy. What if she's gay and the one friend is a girlfriend etc. etc. There are lots of things an unsupervised teen can get up to. drinking, smoking.
My daughter can only have one particular friend here while I am away. And I never leave them alone to go out of town. This is a long-term friend with a mother who is also careful with supervision. We spend a lot of time with them making sure they have good fun, that can't get them into serious trouble.
I am really starting to trust my daughter again. She's a good person who made a mistake.
She now has a boyfriend. He visited yesterday. Boyfriend's mom called to make sure they wouldnt' be alone and I was ok with him visiting. I assured her I was on the same page and that he would always be welcomed but they would never be alone.
These two kids have placed you in a very uncomfortable position. I would probably talk to them about this and take the opportunity to make sure they were being safe. I don't know if I would tell the parents. It would all depend on the kids and parents and if there was anything to be gained by doing so.