we don't have guns readily available in the UK so we don't have mass shootings in schools. The US has a serious problem with mass shootings. This does not happen in any other country in the west
You did not understand, but am not surprised at all.
Why the figure of 100,000? Because social scientists and statisticians agreed that with the concentration of 100k humans anywhere, we are compelled to interact and begin to produce meaningful things, such as science, agriculture, arts, politics, anything. Including killings.
The reason why these shootings usually received so much attention is because of the gun, nothing more. So when you look at the 'Intentional Homicides per 100,000' statistics, the US is actually quite safe despite the amount of guns and ammo per capita.
worldpopulationreview.com
Gang related shootings are anomalies in the US, but gang related killings (not necessarily by guns) in El Salvador is common. So statistically, you would be safer in the US at %90 of any state as long as you stay out of those anomalous areas, like the ghettos, for example, where it does not matter if there is a gun or not. If a gang member want to kill you, he will find a way.
Likewise with mass shootings. Schools, markets, or shopping centers, these are places where people go to relax or do something not related to survival, hence, they are attractive to killers wanting to make names for themselves. You have to look at the individuals, not the guns, not the social environment, because like you said that guns are readily available. If the gun is truly a contributing factor that 'Intentional Homicides per 100,000' statistics would be much higher.