It's taken me awhile to figure out what exactly to do with those pesky runaway teens! I know a lot of Simmers here have the same issue... can't find the hack to keep them at home! So, I added my own cheat for you in case no one else has here.
Try this: Save-a-Sim. For anyone who's not familiar with this concept, follow the below instructions:
When your Sim teen announces she's going to run away, hit Shift/Control-C. Write in the following cheat "move_objects on"
Switch over to Furniture. Grab a hold of the "runaway" teen.
Delete the teen.
Switch over to Neighborhood. And yes, you're going to want to SAVE!
Come back to the lot and your teen is a brand new Sim. Fully happy and all their needs are met! Happy Simming!