In the past decade there have been fifty shootings at high schools in the USA; that is literally just at highschools. In the UK there have, as Marc pointed out, only been 3 massacres in recent history.
And in terms of size our population's about a fifth that of the US; 3x5 = 15 shootings in recent history if the US had had similar circumstances to the UK. And you've had fifty in highschools alone.