April Fool's Day!

Today should be interesting!  The older girls have been talking about April Fool's Day for over a week and what they are doing to fool their parents.  Elyse is planning on freezing her dad's toothbrush in the cup to surprise him in the morning.  Last year she had put a plastic spider on top of his underwear and really got a great reaction the next morning.  She seemed very excited about doing it this year.
I need to come up with something to "fool" the students tomorrow.  Something that no one will get hurt but everyone will have a good laugh about.  I'll think of something!