I doubt that my 15-year-old daughter has a boyfriend in school, but she is hiding it from me. While don't have a problem with it, am worried she might do some wrong things due to lack of guidance. how do I break the ice with her?

You need to speak to her and confirm your doubt. It is a very delicate situation and you don't want her to get taken aback and not speak to you on the issue. Re-assure her that this is a normal phase all teens go through and you are not annoyed with her at the same time you want her to know her what is acceptable and not. Guide her accordingly.