Almost made it

Efforts to avoid classwork increase in direct correlation to the temperature.  A student of mine, H, attempted to leave class having written and signed a pass for himself (without permission, of course).  I intercepted the pass, though even if I hadn't, I'm quite sure his attempt would have still be unsuccessful:

"Pass to the labbarary for H and D"

After I confiscated the pass, he made another attempt, albeit a cryptic one:

"Door

Don't go out"

I'm not really sure.