The Government has published advice on what we should do in the event of a firearms or weapons attack in a public place, run.
That's good to know because we may have tried to engage them in meaningful dialogue but if the Government says just beat it the hell out of there, i would go with that.
They also advise that if running isn't an option, hide behind a heavily reinforced wall as as bullets can go through brick, glass, wood and metal. Lock any doors and silence your mobile phone.
With the running part, i assume you adopt a zig-zag style run until you find a heavily reinforced wall whatever one of those looks like.
The bottom line is leg it and keep legging it until you can't hear the gun fire anymore.