Can't describe things as if the character has never seen them before.
It's the middle of the school year, even. He won't be new to his duties. . . hmmm. . . unless he's installed in his post in the middle of the year. Given that he (and the girl) are only the junior-most of the four sets, that might even make sense.
Some things can be explained to scholars who were never at school this long before the term actually began.
And some I'm just going to inclue more subtly.
It's not for nothing that a lot of stories start with the character arriving.