The statement "playing violent video games in childhood is associated with mass school shootings" is misleading, as there are no studies linking school shootings specifically with violent video game usage. The linked CNN fact-check article explains that the idea of violent video games causing school shootings is a distorted claim by people attempting to blame violent video games as the cause. These claims are based on research studies that have linked violent video games with aggressive behavior, but these studies have never singled out school shootings specifically.
https://www.cnn.com/2019/08/05/politics/violent-video-game-shooting-fact-check/index.html
One of the major sources for the claim that violent games cause aggression was the American Psychological Association, which made the claim in 2015 as described in the CNN article. The APA has since made a 2020 revision to their 2015 statements, stating that their research "should not be misinterpreted or misused by attributing violence, such as mass shootings, to violent video game use."
https://www.apa.org/about/policy/resolution-violent-video-games.pdf
Rating: Misleading / Likely Misinformation
There are a plethora of scientific studies that both support and deny the idea that violent video games are a cause of aggressive behavior. The topic is heavily contested, so it is hard to make any concrete claims on the matter. I think it is worth noting that Harvard Health has quoted the PEW Research Center's 2008 study on video game usage, which found that 97% of teens aged 12-17 played video games, and of those, 66% played games involving violence. Millions of teens play violent video games, making violent video-game-playing school shooters an extremely small minority. If violent video games caused shootings, there would likely be more shooters based on the amount of violent video game players.
Here are the Harvard Health and PEW Research articles. If you want to read more about the claims made by studies supporting both sides of this debate, the Harvard Health article goes into details.
https://www.health.harvard.edu/newsletter_article/violent-video-games-and-young-people.
https://www.pewresearch.org/internet/2008/09/16/teens-video-games-and-civics/