"So that includes not only mass shootings like the one in Florida," said Kertscher, "and instances where there is gunfire outside of the school, maybe in the parking lot - even accidental shootings and suicides."
PolitiFact Wisconsin says there have been three mass shootings this year in the U.S.
As for the 18 school shootings over the last 20 years in the rest of the world, PolitiFact Wisconsin says there's no way to know.
"There is research to indicate that there is more gun violence generally in the U.S. but there's no count of school shootings around the world using that broad definition," said Kertscher.
About 24 hours after posting the tweet, Greenfield took it down.
PolitiFact Wisconsin rated this claim Mostly False.