According to the Washington Post, the number is fake. It was spread on social media as propaganda from a non-profit organization.
Source: No, there haven’t been 18 school shootings in 2018. That number is flat wrong. – The Washington Post
This post is not about pro-gun or anti-gun issues but about the use of social media for propaganda efforts.
A number originating from a non-profit sounds legitimate – but its actually using the “Appeal to Authority” form of argument combined with lying.
Within a short time interval, this fake claim spread rapidly on social media and became a “fact” that professional news reporters picked up and reported.  Once a fake “fact” is published by the professional news services, others will then use that as verification that the claim is true.
From a propaganda effectiveness perspective, this gets an A grade.
It is perplexing why the group chose to exaggerate their count as there are sufficient numbers of shootings to make their point without resorting to being misleading. One would think being misleading would lead to subsequent distrust.
However, remember that in propaganda messaging, the first message people hear is the one that “sticks” – even if subsequently shown to be untrue or misleading. This is why this technique – exaggeration or misleading information – is very effective as a form of propaganda.
When combined with social media sharing, false claims can be widely distributed to the point they turn into “facts” that stick in the mind of the target.
Update: From the comments to the WaPo article, many suggest its okay to be misleading if it leads to someone’s desired conclusion. Or something.