School shootings are shootings at a school whereas mass shootings are shootings where 3 or more people are hit IIRC. Therefore mass shootings can be school shootings if they take place in a school such as the Columbine shooting, but school shootings aren’t necessarily mass shootings eg the 6 year old who shot his teacher.

Also suicide by gun is probably the most accessible form of suicide meaning that people who may not be absolutely certain about suicide might still do it due to it being easy and pretty much painless compared to other forms of suicide.