I'm running a game that takes place in a school and my players want to prank the principal.

I'm trying to figure out the best way to run it as an encounter. I'm planning on have the door to the office be locked and there being traps. I was also going to use things like animated armor, a rug of smothering, and flying swords in the actual office to make it a combat encounter. 

Does anyone have any ideas of other ways that I can make it a bit more of a challenge/more exciting for them?

There should be some kind of magical trap - perhaps a Glyph of Fear?

Another option is to have the surprisingly effective 'weak' guardian.    Perhaps a pet gerbil that turns out to be an awakened gerbil that has levels of Sorcerer.    And throw in a "Judge me by my sized do you?   Feel the wrath of BOO BEAR, KING OF THE GERBILS!"

Treasure:   Have them discover a Secret.   Something that could be misinterpreted.   Is the principal working for the Mindflayers?   Of course not, he escaped from them long ago and hates them with a passion.

Well, the principal of a school of adventures is probably good alignment, make traps non lethal

maybe a war forged that was shut down can be activated, and was a school security guard, with a couple levels of fighter     

Maybe something like a giant mechanical thing made of books. That sounds painful, because papercuts.

how old is your group? I suspect not very if they care about the principal of a school.  You may be setting yourself for a scenario where a bunch of IRL kids laugh and joke at how they stabbed the principal - you've already said you're planning a combat scenario in a school so at the very least you are responsible for planning a very detailed scenario where a bunch of kids have to fight and hack their way through a school.

yes its all pretend, but you should ask yourself if you really want to go there.

One of my favorite traps was made by a very devious friend of mine for another system.  It involved three heads which rapid-fired three spells off, a half-second apart. 

The first cast a spell to cause the target to drop anything they are carrying (typically their weapons). 

The second case a fear spell, causing them to run away as fast as they could.

The third cast a forget spell on them which made them forget everything from the past 5 minutes. 

So... They have lost their weapons, they're running away, and they don't know why.

I don't think the spells exist exactly in DnD but you could always improvise.  It sounds like the kind of thing a headmaster who's (probably) had students prank him or her before would set up.  Non-lethal, and they might even leave some evidence behind to have to come and collect. . . 

Eric the Grey 

If you want to have an aggravating combat where you as the DM is laughing, add a fairy dragon or two. This should take care of things.