I teach 7th grade Math and a couple weeks ago before class, a student took some of my hand sanitizer and squirted it on a student's chair. I asked the class who did it but of course nobody came forward. I then told the students to approach me after school if they know who did it as somebody had to have seen the student. Well nobody spoke out and today when the students came back from their break a student complained to me that someone had put gum on her chair. Once again nobody saw anything. How should I handle a situation like this where someone is doing something and nobody will speak out even though they must know?