I live in a HA property that has a communal garden behind our garden. Recently a couple of teenage girls have been hanging out on a quiet part of the garden that is just by our fence. I noticed a lot of loud laughing.. all fine I thought until I realised they were doing laughing gas. Leaving all the used balloons on the floor. Last night I went a had a word asking them to move on. They went without much fuss. I wondering now whether to report it to HA -I’m actually more concerned about their health than noise. They seem around 15/16 but hard to tell.