If you make a change in only one school, you end up with selection effects where interested parents move their children into (or away from) catchment areas based on vibes.

Then you can't really measure outcomes, because the strongest predictor of student performance is parents interest and resources.

You also run into issues with teaching skills and standards, you need a high level of planning and adherence to the supplied plan in order to measure outcomes; otherwise it's just vibes based on individual teachers.