She shot him a look at the way he said move on; her expression wasn't quite warning, but there was the tilt of an eyebrow, a questioning glance. There were so many jokes caught behind her teeth, but she still couldn't bring herself to joke about it just yet. About wishing death on a professor, even if Binns' card had been punched long-ago. It still felt too difficult to broach in the wake of that war, even for these two, who had grown up with death's long shadow looming over them.

So. Instead, Luna watched the unicorn, and when it came to her, she folded a small slip of a note into it to carry back to Harry's desk. A quieter, slightly more discreet way of communicating:

if i pretend to faint do you think they'll let us go to the 'infirmary'