Your understanding of the trend is flat wrong. Check out sources via your favourite search engine to see the actual trend of school shootings in the US. (and then check them out for the developed world, too if you want your eyes to pop)
I mean, what exactly are you suggesting? That these school shootings are democratic plants? Respectfully, I find that exceedingly hard to believe.
Mass shootings and stabbings happen all the time. We only hear about them nationally when it's convenient.
Most likely it's selective reporting, but I wouldn't dismiss the more grim explanation For example Axios (which should be called Anxios) devoted 4 posts out of 20 on its front page to gun control. On other days it looks similar. If forgot to report that today 100 people died in car accidents, 5000 died from cancer and heart issues, a few multiples of that went bankrupt from "treatment" that disn't cure their cancer, but made disabled and miserable. Yet they devote 20% of their attention to a microscopic issue that coincidentally builds political capital for one party.
I would love to see a chart that supports your assertion.