The project is quite cheap, you can do it find almost only things that usually you can find in a school or free.
A big cardboard box, the shape would be more or less the shape of your classroom.
Boxes in various dimensions
Shissors, cutter, felt-tip, crayon, pencils
There will be some other materials, depending of what you'll be put in your diorama, like modelling clay, little caps, staplers, fabrics, velcro.... use your teacher-fantasy!
Take the measurement of the classroom with the pupils: note that the classroom is a square, or a rectangle, like the box you chose.
Cut the upper lids of the box.
Cut the windows and put a sheet of transparent plastic to make the glass.
Paint the walls or glue some papers to them: given that my classroom is pink the lower part and white over the 1,50 m, and the box was brown, I've done it with white and coloured paper sheets.