The Map Is Not The Territory by kitsune13
It's a Will/Rachel fic, which may not be everyones cup of tea. However, they are my super sekrit Glee 'ship (mmm.... maybe not so secret anymore!) and kitsune13 does a great job of showing the progression from teacher/student to friends to something more. It's an amazingly well-written fic, her characterisation (Rachel, especially) is just spot on.
Cannot recommend this highly enough!