Step 1: A person's proportions are easy to draw once you understand that halfway up their height is where their crotch is. The middle point.
Step 2: When drawing people on a background, you need to figure out where the horizon is. In this case, I had to figure it out using the existing drawing. The red lines show my process. Where the lines intersect is where the horizon (blue) line is placed.
Step 3: The height of a person near the doors can be guessed. Most people don't hit the doors they walk through, so the average student will be shorter than the door, but taller than the handle. The green boxes I made are not the be-all-end-all as once we duplicate these body proportion boxes, we'll alter them slightly to create students of different heights.
Step 4: You'll notice my green boxes have diagonals on them that is how you determine, without a ruler, the middlepoint of a square object. Where the lines of the diagonals from each corner intersect is the middle. (Or the crotch of a future person)
Step 5: Placing a random X on the horizon (pink), create lines that touch a green box's corners. Once you've done this for all 4 corners as I did, you can connect the extended lines with horizontal lines to create a box. Then use the X technique to determine the middle (grey).
So that is the technique to A) proportion a human body and B) size bodies correctly on a background.
ACTION STEP: Create 15 boxes sized properly to represent people we'll draw in later. Remember: the pink dot on the horizon can be placed anywhere along it. Move it around. Also, feel free to expand the canvas to allow yourself more horizon space to put your pink dot on.