Please check out the following paragraph.
Then I admonished him, "Don't you see the sign saying 'Teacher's Washroom'? You students should obey school rules and guidelines. You should go to students' washroom. Never come here again."
'Didn't you see the sign?' as he's already in there.
'Can't you see the sign?' if he's about to walk in.
The sign would say Teachers' Washroom unless there was only one teacher.