So the leaving-do was a lot of fun. Mostly because people were inhaling laughing gas. I kid you not. Here are some photos.
Apparently it's the latest craze. You use a canister like this, that you can buy on the internet.
The cartridges that you put in it contain nitrous oxide. They are designed for making whipped cream.
It's completely legal. Apparently the high lasts about a minute, but I didn't try it because I'm a pussy.