First, is a Student Shadow Study. Originally developed by NASSP a shadow study charts the experience of students throughout a school day. Observers follow randomly selected students and record the ebb and flow of activities every five-to-seven minutes. Of course, students quickly figure out that something is going on. I suggest talking with the student you shadow before you begin and explain that you are not evaluating them or their work. It's also a good idea to talk with the student at the end of the day to gain additional insights into their experience.
Shadow studies work best when several observers collect data by shadowing students. The December 2009 issue of Principal Leadership provides more detail about this approach.
The second technique is to conduct a series of focus group meetings with students. A focus group is a set of people brought together to participate in a guided discussion about an issue---your school. While not a formal focus group, a principal I met in North Carolina invited groups of students to meet with her during lunch. She asked students to tell her about their school and their classes. She listened and looked for patterns among the students' responses.
Students are incredibly insightful and are able to provide useful information that can be used along with other data to improve schools. I recommend both strategies to you.