I have walked many a class of students down the hallway. The expectations are always clear and reviewed just before we leave the room. However, with some groups, it is still quite a challenge for them to be quiet in the halls. So here is something I have tried that seems to help...
Before we leave the room, I review ONE math fact. 2 + 3 = 5, 9 x 7 = 63, 8 - 2 = 6...I say it, they repeat me, I call on random students to recite it to me...After about 30 seconds of this, they are told to concentrate on that fact until we get where we are going.
When we get to our destination, I go through the line one by one (often as they are entering the classroom) and ask students the fact OR an extension of the fact--depending on the student. So some students are asked 9 x 7, some 90 x 700, some 63 / 7, others 63,000 / 90...If the student gets their fact correct, they are good to go into the classroom, if they don't they get another chance.
It is a pretty silly thing, but I do find that if they have a fact to concentrate on, they are quiet in the hall. I even tell them to repeat the fact to themselves as we move through the hall. :)
Give it a go! I hope that it works for you as well.