I don't recall reading/hearing about 18 school shootings in the US in 2018. Do you? How do "they" decide which to publicize? # of Deaths?

Because there have not been 18 school shootings this year. 
It is a lie. 
Is a man committing suicide at 2:00am in a school parking lot a school shooting? 
Is a man firing a gun in the air of a school parking lot at 1:00am with no injuries a school shooting?
Is an accidental firing of a holstered gun my a police officer at school with no injuries a school shooting?
Or a random bullet fired on campus with no injuries and shooter never found a school shooting? 
Or an accidental shooting at a practice for a school shooting team that was not at a person and no injuries, is this a school shooting?
These are and many others are counted in this 18 school shootings this year. 

To myself and I believe most others a "school shooting" is when someone goes to a school with the intentions to shoot children and/or administration at the school. 

That is why you did not hear about them...they were not school shootings.