I'm thinking of something silly...

You would think so but no, usually they just hang around the house until you get a notification saying something like "its getting late i should really be leaving" or something like that. I don't think the game registers that you have another teen on the lot that should be going to school. I wish , when you have only one household sim on the lot and no other sims, that guest sims would leave when your sim goes to sleep or leaves for work too like in previous games in the series. Guests normally hang about my house using the pc or whatever stopping time from speeding up.