When you're on the bus and the stalker texts you, you need to text back. Then, when you hang out with Allie (at the mall) or with Adam (at the arcade), you need to show them the weird texts you keep getting. At this point, you'll have the option to let them call the stalker, which you need to do to get them killed.
Also, I don't know if it changes the route or not, but I think you need to post the rose on social media for the deaths to happen. I've played so many routes, I'm starting to get them mixed up lmao.