'sono' is both first person singular (I am) and third person plural (they are). Io sono un uomo. Loro sono belle (they are beautiful).
Though I suppose, since Italian doesn't have to have a subject pronoun this could technically be:
[loro] non sono i tuoi pantaloni; they aren't your pants. [io] non sono i tuoi panteloni: I am not your pants.... a fairly award situation indeed.
"why are you wiping your wet hands on me?" "I just washed them and could not find a towel"" "I am not your pants"
However, I can't fault the grader for not anticipating that situation.
English used to use the singular: in place of trousers, you could say "he wore a grey flannel trouser". We old Wodehousians are familiar with this. I like the fact that 'trouser' has in recent years become a verb in English, as in "he trousered the five pound note". I wonder if you could do that with 'pants'?