Are they contracted to work there til a certain time? Do you tell the teacher ahead of time or send in a note saying you will be stopping by? The few times that I've done this I feel like they are rushed and I'm keeping them from getting to wherever they need to be. Plus there are random meetings scheduled, or they are on bus duty that day, or something else.
For drop off, parents can't enter the school building unless they sign into the office, show ID, get a name tag, etc (big hassle). And then there is the issue of a bunch of kids being right there and the teacher attending to them while you are trying to talk.