Could you ask another member of staff to pop along at the end of the lesson to 'give you a message' and therefore catch them in the act? The they do the whole "Oh Hello James, did you need to ask Lionkitty a question? But it looked like you were opening the door again just then? Are you sure? Did you want James for something Lionkitty? Well, off you go then James Smith." If it ever happens again, you know which boy to call for at break time. Chances are it wont as he knows hes been caught out.