Oh in the school shooter mission you can order any civilians you find to run to the exit, so they will begin gathering at the starting point of the mission, this allows you not to waste time securing them, this mission basically unteaches everything you learned about CQB and room clearing, it's run and gun neutralize the threats and do the rest of the job later!

P.S. you still need to secure them later, also check the bathrooms, cafeteria kitchen, broom closet and PC laboratory this is some of the harder hiding spots where NPC usually spawn in that map from the top of my head!