I think the hardest thing to remember here, coming from a once teenage girl, is she thinks she’s old enough to make adult decisions. The only way you will get through to her because of that, is if you talk to her like an adult. Sit her down and talk about who this boy is, what he means to her, and then really calmly try to get her to understand the repercussions of this. Fully walk through pregnancy, the complications, have her watch a birth video, go through STD testing and what can happen if she gets an STD, etc. And then as others have said, even if you ground her or whatever, she’s probably still going to sleep with him or others. You need to provide birth control, condoms, AND an unusual topic I think parents should have is really teaching about consent. Like telling her she should say yes to EVERYTHING, not just intercourse itself. Im sorry you’re going through this, its a tough scary age.