I've been trawling the net, but can see that type of massacre (schoolroom I mean) in UK.

I think here it's more knives than guns amongst teens. Last year 26 teenagers were killed in stabbings and shootings in London. http://news.bbc.co.uk/2/hi/uk_news/england/london/7172981.stm

Not sure what the quota is for the whole of UK.