Let them stay for the "things to do" as much as they want and let them know that you will assist with tasks during your conference time. Keep it professional, but keep that boundary.

I have been a HD for 13 years and never have I ever required assistant directors to stay as late as I do. If I stay late, it's my choosing. Most days, I'm out the door 20 minutes after my sectionals end and that's just me getting my office straightened up for the next day.

When I was an assistant (young, ambitions, no family), I was fortunate to work with a head director that would tell me to go home when, or before, he did!