I afford all of my students a degree of trust from the beginning, it is their decision to be responsible with it. Students that are off task or don't follow posted classroom policies are subject to repercussions, such as a reduction in classroom participation grades, alternative assignments, or removal from the main ensemble to focus on solo repertoire.
I am a proponent of the Suzuki method, in which students immerse themselves in music to grow as musicians. I also foster a productive, musical environment in the class that students can feel free to make music in.