Everyday you must line your students up before going outside, going to lunch, etc. Why not use this time to teach place value? All you need is a good system. Take a look at this place value system aside of this line of students and you should get a pretty good idea of what I have in mind.
Now, the trick to this exercise is how to 'place value' a classroom of 25-30 students. Unless one is inclined to venture out to the trillions or trillionths, it would probably be best if the instructor used multiple lines or one large single-file line with multiple systems of place values.
let us assume you have five rows of five student desks in your classroom (of which I will show two methods) depending on how you line up your students.
1st: How do you line up your students? Do you call them by row? By name? All at once?
If by row: I would suggest labeling each desk tens, tenths, etc and calling them up in one or two rows at a time. Leave it up to the students figure out which one of them should be first, second, third, etc. You could also hand out index cards with names such as “tens place” and a pic of where the “tens place” is located.
If by name or all at once: I would either hand out index cards with names such as “tens place” and a pic of where the “tens place” is located or I would label the floor next to the door (or wall) with place value names and tell them their place value as you call them up.
“Lets get ready to line up to got to lunch”
“We will line up based on name and place value, the biggest place value is lined up first”
“Billy and Stacy you line up first, Billy, you are the “tens place”, Stacy you are the “ones place”.
--This forces Billy and Stacy to discuss who is first in line--
“Joey and Amy you are next line. Joey, you are in 'tenths place', Amy you are in the 'hundreds place”
Note: If your students have never been introduced to the place value system, you cannot expect them to know how to arrange themselves. Thus, a poster on the wall may serve well