If I were OP I would go to to the office that provides permission for elevator usage and tell them that "I was late to class because unofficial people were using the elevator. Unless they had an invisible disability... though they looked mobile enough to climb one flight of stairs" (The last part is a lie obviously; but hayeeeeeeees) "I'm just concerned people are taking advantage of what should be there for people who really need it" That would hopefully get the butterball rolling, so fatties will have to climb the stairs long after OP is well enough to go up them skipping every second step!