Laws vary from state to state in how they handle adolescent rights to decide their own health care.
Having said that, I believe that the counselor must proceed judiciously in sharing the information. If there is reason to believe that the girl's parents would harm her if told, the counselor needs to take steps to insure her safety. I suggest that the counselor first talk with the girl about the necessity of disclosure, for her own benefit as well as that of her parents, the boy involved and the unborn child. Ideally, her parents, or both hers and the boy's parents could come to the school immediately to begin the conversation, with the counselor present. Chances to preserve the student-counselor relationship are greatly enhanced if the counselor can be part of the initial discussion.