Were the attacks on the Muggles at The Quiddich World Cup pre-planned?

I'm wondering if the attacks on the Muggles at the Quiddich World Cup were pre-planned or just a spur-of-the-moment thing. Everyone had their uniforms which would lead me to believe it was indeed pre-planned, but it doesn't seem like the sort of thing they would do because it isn't a very good idea. As Mr. Weasley and Hermione suggested they could have been drunk. So does anyone actually know if it was pre-planned or have any evidence to whether or not it could be?