Locking the door during all class times and only opening it during passing periods was something I tried last year, but that means any time a student is late, I leave class unsupervised to unlock the door. Any time a kid is leaving early and needs to grab an instrument, I have to leave class. When I have morning duties, kids can't put their instruments away and that puts me in a bad position with other teachers who then have to deal with horns in their classrooms, gym, and cafeteria. As the campus music teacher, I have extra duty in the mornings almost constantly.
It came to a boiling point yesterday when a non-band 6th grader was found hiding under my desk to skip class. I don't even know this kid and an administrator found her under my desk while I was running sectionals in the next room. Now, I look bad and my room has a rep as a place to get out of things.
My next idea is a security camera. I thought a baby monitor might work, but I need something that at least lets me go back and look at things. Any of you use something like this? They all seem to need to run through wifi.