Sixteen is very normal to have sexual relationship these days. But parenting is tough. I can't feel right to say what is right or wrong for you to do. KianaPrice1D is right. If you can talk about sexual relationships with her that would really help. You don't have to tell her anything right now even, it is enough to assure her that she can come to you for any questions about that. That way also you would know what they are doing. It is better to know things. Also (I'm a guy), it changes a lot if the boy knows that you know. He will behave himself more, be more careful. Otherwise boys can get more like "I can do whatever I want with this girl, no one knows anyway".
Look like you needs to have a talk with here daughter. When your daughter is alone talk & Explain your Concerns about here. oh & just do let you know I think the hole dating thing in public schools is all mess up as you can see.