well that's an awesome opportunity.

Very nice.

I like the ideas of guerilla art, and guerilla gardening, so let me give some advice/knowledge that might help you out.

You might consider a light mist from a spay bottle of water to make your chalk stick, keeping slightly better definition (if desired) when you lift your stencil off, and adding a little more longevity to your art.

Quick chalk/construction lesson for anyone interested:

Hardware stores especially the bigger contractor oriented ones sell powdered chalk. You can buy it in sizes and bottles like your old fashioned ketchup or mustard squeeze bottle, or in sizes/shapes equivalent to the costco/price club jumbo size condiment squeeze bottles.

The most easily found are blue, red, black and white.

Blue fades the quickest, which is useful for things that will get painted or where you want to be able to wash it off or simply don't want the chalk to transfer from one surface to another.

Red is more durable and harder to clean off and can transfer.

Black is not really chalk and it contains graphite, among other things IRC, and is the longest lasting. It transfers. If you're not careful it can be like cancer; once it starts spreading, you can't stop it.
Black and red require real scrubbing when you get them on your skin; especially black.

In concrete forming we tend to use black and red, but we will use blue chalk on the surfaces of the forms if the concrete will be left exposed ('architectural'), this way if chalk transfers from the plywood to the concrete, it will be more easily removed and/or less noticeable.

I've only ever used white chalk once when I had to cut out some relatively fresh asphalt (the other colours don't show so well on the black surface). I cut up those chalk lines, so they didn't have a chance to last so I can't tell you anything about durability, but I assume it's like the blue, because it's "real" chalk.

When we need chalk lines to stand up to the weather (longer term layout purposes), we use a clear marking paint (spray can that works inverted, usually in day-glo colours) to seal it (or we use an acrylic sealant with can spray can like they pest control people use). But that might be bordering on vandalism.

If you need colours not found in the hardware store, you can buy buckets of chalk 1" diameter in assorted colours, in toy departments, this chalk may get you in less trouble than construction chalk and it's probably much less durable.

If you wanted, you could use something like a coffee grinder, blender or food processor to powder chalk sticks (start with a 'crush ice' setting) - maybe fold them in a cloth and break 'em up a bit with a hammer first.

Oh. So simple yet effective. By the way the stencil's uber cool.