In order to obtain this achievement, you'll have to
convince Michael of staying in town in the past time scenes. Doesn't matter if you convince him of breaking up or staying with Clarissa, that doesn't affect the achievement, but it does affect what you'll have to say to him to convince him. I'll explain this now:
During the three past Michael scenes, use as much related dialog options as possible. Tell him that
you'll miss him,
question his motives, and, most important, in the last scene, when
Michael and Alex talk at home and he asks for your opinion, you'll have to tell him the following, depending on if he and Clarissa are still together.
- If they are still together, tell Michael
"Don't go"- If they broke up, tell Michael
"Go to school here".
Also, you can combine this with your "The strong, silent type" playthrough, as the fact of don't talking to Michael at all will make him assume you want him to stay.
IMPORTANT: This achievement is compatible with every other except for "New Beginnings". If you try to get both in the same playthrough, you'll only obtain "Thicker than water".
If you meet these requirements, you'll obtain the achievement during the "stat count" before the credits.
For a guide on how to get all the ending achievements in the lowest number of playthroughs, click here