Bully

This achievement is for spending 5 hours out after curfew and should unlock as you progress through the game normally. However if you're impatient (like me) just spend 5hrs out of your dorm between 11pm and 2am when you pass out. This only allows 3hrs at a time, so you'll need two nights to do it.
Also, try staying somewhere prefects won't bust you, I went behind boys dorm.