Middle School teacher here. I think it is also important to note that in the OP's experience, it was the school, not the teacher, who requested no drop-in's. I am not a fan of drop-in's unless it has been pre-arranged. I have been, too often, caught off-gaurd by a parent coming into my room to bring Johnny his backback and then want to talk to me about grades, family issues, curriculum questions, etc... I am a great teacher and even I can't keep 6th-graders on task while conferencing with a parent. It also creeps me out that strangers are wandering the campus with no pass or escort.
Did any of you see the moive "Waiting for Superman"? THere was one family where the mother sent note after note and left message after message for her child's teacher, and it was never responded to. SHould that mother not be allowed to just drop in and see what's going on in the classroom? There are some teachers that SHOULD be dropped in on.