I am currently watching the British series Utopia and I a was curious if someone knew or could point to a reference on how do they make those ultra realistic shootings and deaths.
The sequence in question was filmed at Alsop High School in Walton, Liverpool during the summer Holidays where none of the actual school children were present.
England has some of the most most stringent gun control laws in the world, but there are no restrictions to shooting in schools as long as they meet the safety criteria.
The school has not, and is unlikely to comment on why it chose to allow the series to shoot at this location, but it is almost certain they were aware of the scene's content.
Beyond this, it's just quite a conventionally shot scene, without any elaborate Special effects.