LOLLIPOP CHAINSAW RePOP

Quite hard to miss this one unless you let the 3rd classmate die. As you progress through the prologue you'll come across 3 people that need rescuing from zombies. The first 2 will be rescued as soon as you kill the zombies surrounding them, the 3rd classmate requires some escorting and protecting until the helicopter rescues him. As soon as he's rescued the achievement will pop.