Who gets the small class?
Every kid has two potential outcomes: a score in a regular class (y₀) and a score in a small class (y₁). We only ever see the one for the road the kid took.
Every run so far: same kids, a new draw of who gets the small class each time
Made-up data for teaching. Both potential outcomes are invented for every kid; the six students' observed scores are the slide's, and their other roads are set so the small-class kids would have averaged 89.