Chalk on the ground

I remember drawing pictures with chalk many years ago in primary school. Not on the floor of course - our teachers would have had a problem with that - but rather on large pieces of cardboard.

When I saw these guys I wondered if their time wouldn't be more well spent doing the same with some kind of super-hardy paint. The chalk will wash away with the first, or perhaps second, rains but having these kinds of paintings in appropriate places could be quite a feature. Don't you think?