Alarm Clock Switcheroo

Set your alarm clock to wake you up at about 3:00am on a school day. Walk into someone's room and set the clock for 12:00. They will think that it's very late.

Leave No Clue
Don't set the victim's clock for exactly 12:00, set it between 12:00-1:00. But don't make it a number that starts and hour or starts 10 minutes. Set it at a time like 12:09, 12:54, etc.