The Janitor cleverly glamors his handwriting so that (hopefully) upon reading it, JD will either see it as Jack Harkness's handwriting, or as vintage typewriter print. Then he slaps a ward on it so that, should anyone else read it, it'll turn into a bottle of bleach.
JD-
Meet me outside by the Quidditch field, near the bleachers ASAP.
It's urgent. I'll be waiting.
-Jack