Intercept the note

You move swiftly and grab the note, just as it is about to change hands again. It is folded up, and you can't tell what is written on the paper.

You could just throw it away and move on, demonstrating that you aren't riled by such antics, or you could open it and read it, so you can find out what is so important to your students that they would risk getting in further trouble. Then again, it might be fun to find out who wrote it and have that student stand and read it out loud in front of everyone; that would really show them a lesson.