I agree, there's no standard definition. I grabbed the 8 / 43 number for 2019 here [1]. I see they have similar numebers as what I found for 2018 as well [2].

The 2018 numbers I was referencing was from this 10 year overview [3].

[1] - https://www.edweek.org/ew/section/multimedia/school-shooting...

[2] - https://www.edweek.org/ew/section/multimedia/school-shooting...

[3] - https://www.cnn.com/interactive/2019/07/us/ten-years-of-scho...