Four students - Justin Shilling, Madisyn Baldwin, Tate Myre, and Hana St. Juliana - died.
Six others, as well as a teacher, were injured.
Ms Crumbley and her husband are the first parents to be charged in a US mass school shooting.
The boy’s father, James Crumbley, will go on trial on 5 March.
Ms Crumbley, 45, took her son, Ethan Crumbley, to a shooting range days before the students were killed.
In December, the shooter was sentenced to life in prison without parole.
Prosecutors allege that Ms Crumbley and her husband gave Ethan access to a gun, ignored his signs of mental health struggles, and refused to take him home from school when they were shown his violent drawings on the day of the attack.