The term in loco parentis, Latin for "in the place of a parent" refers to the legal responsibility of a person or organization to take on some of the functions and responsibilities of a parent. 

Would seem to me that a school that assumes the right of 'in loco parentis" also assume the responsibilities of "in loco parentis" and, thus, is responsible for the safety of the child/ren. If they fail in that regard, to protect the child/ren, they are culpable and liable for damages.

I think the parents of all school shooting victims have standing to pursue remedies, individually or through a class-action lawsuit