Not Legal Advice: Assuming you intend to do this to a car that you actually own, there are a couple of simple methods to flattening a tire for something like an excuse to stay home from work.
You can get a Valve Stem Remover from stores like Walmart for around $6.90; twist the valve stem until air begins to seep out.
If you have co-workers that ride with you and want them to witness a problem you can lean a nail into one of the tires so that as you drive over it the nail is forced into the tire causing it to lose pressure.
Note that these tips should only be used on your own personal property, not someone else's, you may find yourself being arrested if you attempt to flatten the tires of someone else's car!
In fact in the news, reported by USA Today, the sons of a first-term congresswoman and Milwaukee's former acting mayor were charged with criminal damage to property for slashing the tires of vans rented by Republicans to drive voters and monitors to the polls on Election Day. These are felony charges that carry a maximum punishment of 3.5 years in prison and a $10,000 fine. That's not something worth risking for a prank.
If your tires are going flat, the first thing to check, obviously, is whether they have any holes. But if you can't find any and the tires still leak, there could be another answer.