I did this at the high school but I'm sure it would work at the middle school. I actually had a student "steal" a printer from the computer classroom, during class (it could be anything). I distracted the students and then sat back and waited for someone to realize what had happened. We elected police who interviewed all the witnesses (a class of 32). We discussed the evidence and came up with a suspect and then held a mock trial (jurors, attorneys, judge were from another class). It was great and a real lesson on how everyone sees things differently. Each student (approx. 65)played a role so was actively involved. It played out over a week of class time. Hope this suggestion spurs some ideas.