Marley tells Scrooge that one spirit will visit him at 1:00 am for the next three nights, but they all appear to him in the same night. This is repeated verbatim from the book, in which, following all the visits, Scrooge calls them "clever spirits" for doing it all in one night.
When Scrooge re-visits his old classroom; the ink markings and spill patterns are virtually identical on all the desks. The animators more than likely just placed several copies of an original desk throughout the scene. But it is also reasonable that, since it is all "from" Ebenezer's mind, that he was simply recalling how his own desk looked and applied that image to the other desks. There is no reason to think he paid enough attention to the desks of other students to remember how each looked.
According to Scrooge's, "future" tombstone it says he was born in 1786 meaning Scrooge is just 2 months short of his 58th birthday but he looks more like he is in his seventies from his appearance to a use of a walking stick. The reason Scrooge looks all old and haggard is to show that he's aged prematurely due to his vile attitude towards the world.