In 2018 there were 37 kids killed in school shootings. It doesn't specify what kind of weapon. And I am sure they weren't all from what you would call "assault rifles" So I'm sure there is a lesser value than 37 kids killed with an assault rifle but we can use that number.
CNN examined 10 years of shootings on K-12 campuses across the US. The numbers we found reveal some sobering and surprising truths.
www.cnn.com
The government reports an average of 49 people die from lighting strikes each year. I couldn't find the number of actual lightning strikes each year but wikipedia says that 10-30% of the strikes are fatal. So if 49 people died, at 10-30% of the people being struck.. You can do the math. A kid is more likely to be struck by lightening than die in a school shooting.
en.wikipedia.org
Kids being killed at school is terrible and unfortunate. I am sure we can all agree this number should be zero. I am not making light of the issue, but blaming "assault rifles" is fucking stupid being there are 5 to 10 million of AR15 style rifles in the USA not counting AK47 type, and many others like the mini 30 and min15's and so forth.
en.wikipedia.org
So all the numbers are very small fractions of a percent.