This book is not about the Columbine Shooting. It is an entirely fictional shooting that takes place in the fictional town of Opportunity, Alabama. As Marieke Nijkamp has stated in multiple interviews (citations at the end), she did not base this book off of any one shooting. Also, as even some cursory searches on the internet could have shown, there were two shooters active in the Columbine Shooting.
https://emcastellan.com/2015/11/09/a-...
https://www.shelf-awareness.com/dedic...