Contact the college and ask them the dimensions of the standard mattress issued at the college dorms.
Cut a piece of 1/2-inch thick plywood to the dimensions of the mattress. Add 3 inches to the mattress dimensions to allow for extra room around the mattress when it sits on top of the loft.
Frame the bottom of the plywood using 2 inch by 4 inch boards cut to the necessary length. Screw the frame together at the corners, and then screw down through the plywood to secure the frame to the plywood base.
Make four 48 inch high legs by cutting two 4 inch by 4 inch by 8 feet long posts in half.
Attach the legs to the frame by placing them in each corner of the frame and using four screws down through the plywood base and four screws in each of the frame boards for a total of twelve screws on each leg.
Cut 2 inch by 4 inch by 8 feet boards to size so that they go across the legs at the narrow ends of the frame. The boards should go stretch to the outside edge of each leg. You will need three board per side for a total of six. These will be supports for the legs and the ladders to enter your loft.
Attach the supports to the legs at the narrow ends of the frame using four screws on each side for a total of eight per board. Put the first board 20 inches from the floor, the second 32 inches from the floor and the third 44 inches from the floor.
Cut one 2 inch by 4 inch by 8 feet long piece of wood to size so that it stretches across the long part of the frame and so that it stretches to the outside of each leg.
Attach this board across the two legs along the long side of the from 30 inches from the floor. Use four screws per side for a total of eight. This will give support to the long side of your loft.