Take your teen to a free clinic and have her tested STD's. Have the doctor give her a reality check on teen pregnancy and diseases.
Do you know the parents of your daughter's friends? If not, get to know them, talk to them about your concerns and see where they are on the subject.
It's past time to pull in the reins on this child. I would limit her social activities immediately. Have her concentrate on school and family until you feel she can be trusted not to treat her body like an amusement park.