I'd say "the notation x/y means a fraction, and cannot be used for coordinates. Likewise, (x, y) means a coordinate and cannot be used for fractions. They are related but should not be conflated."
That makes it sound way to absolute. You should say something more like "please write coordinates like (x,y)".
Incidentally, in some contexts, (x,y) is how we write fractions (they are just an equivalence class based on an ordered pair of integers).
As long as I am on the subject, (x,y) also means the inner product of x and one. And I had one teacher for whom (x,y) means f(x,y) where f would change more or less every section based on convience.
On second thought, maybe we should be a little stricter in teaching students that math notation is rigid. If we raise a generation of mathematicians believing it, then they might clean up the current mess of notation we have today.
As I once said after someone, after a discussion on some trivial mathematical argument that he didn't grasp, said "OK, but one plus one equals two, right?": "it depends".
