Ball game: "kaleren"

Lately Abbie has been playing a ball game at school with her class mates called kaleren. I have no idea what it is called in English. As a matter of fact it is a game completely new to me as well. Maybe it is something Belgian...

Here you see Abbie, Roos, Dries and Luka playing the game (you need a ball and a wall and any number of players) after school. They have all kind of rules I don't seem to understand, but it is the game to play at the moment.