I believe the school had less than adequate security measures. For that, they are responsible.
I was a PTA president for 10 years in an average size, suburban school district. The district has three K-2 sites, 2 3-5 sites, one 6-8 site and one 9-12 high school. Because of my children being in special dual languge programs I eneded up with kids in alll but one of the lower schools. Buzzed in, sign in, sign out, id stickers were the norm in all of them. Security cameras probably first appeared in the HS and Middle school in 2000, coming to elementary and primary schools later. Attendance policies give HS parents a live person phone call of a cut or absence the evening of the day after it occurs. Middle school parents generally receive a live person call mid morning on the day of the absence. I have only received the occasional call the day of an absence from an elementary or primary school, and it varies by school, despite there being a policy in place. I think staffing played into not receiving calls, as well as the school feeling like they "knew" me. However, when I ran PTA sponsored programs after school (six week sessions of ballet, karate, cooking..) I had to pick up my charges at their classrooms, take attendance, deliver them to their instructor and then scurry down to the office to call a parent if the child was absent. If my student list contained two parent's names - indicating separate living arrangements - I had to call them both - even if the first one reached said "Johnny is home sick". It was the notification practice the district followed.
I can't help but wonder what would have happened if Skyline school had called DY that day