The only explaination I could come up with is it's a classroom full of students, most likely in a computer course of some sort, that are playing in their free time and then they all stop at once when the teacher starts teaching, but that's a bit far-fetched.
Though how they all get around the filters that are surely on this place... unless they really ARE computer students who can get around it all.
Oooh - gone within 15 minutes. The forum logs you out after 15 minutes of inactivity, unless you tick the remember me box, don't forget. Which could explain how they all disappear at once.
That's assuming they all came at once though, which they did, and there's still no explaination for that.
Plus, I used to play games with my friends in school during computer classes, most game sites were banned but there were always some that weren't, and as soon as someone found one, everyone knew about it.
It seems to be a constant battle between the power-hungry admins who control the filter, and the students who can get round it no problems, at our school.
Google is set to Safe Filter On, we can't run applications on our accounts... we can't even right click!
Well at my school it was as simple as using a proxy to get around the web blocker. Until they figured out how to prevent people from entering proxies like 3 years later, it was a sinch. I just ran my own proxy server at home so I could pretty much use my IP at school as a proxy and bypass the filters. Only me and a very select few of my friends used it though, which was great because most proxies are horribly slow, but since mine was running off my home connection it was nice and speedy even with 3-4 people using it at once.