I have recently become much better about insisting that appointments be made when parents just drop by. Never once has a drop-in taken less than ten minutes of my time. And those 30-60 minutes at the end of the day are mighty precious to me. The last time a parent stopped by I simply told her that I would love to talk to her about her son's progress in my class but I had something already scheduled for that time (inputting grades into the computer and then leaving to pick up my own child) and would have to meet with her another time. She said "but this will only take a minute..." I replied "Great - would you like to meet tomorrow at 3:30 or later in the week?" She was a bit miffed but I was able to get my work done on time. If I have the time I will meet. But that is very, very rarely the case immediately after school.