I think it would be tricky, because clearly small classes are going to be automatically better
Clarifying--you meant to type "aren't" going to be automatically better, right?
The FL class size amendment keeps classes at 18 for K-3, IIRC, so they're right around the same size as the STAR project classes. I read some pretty valid criticisms of the STAR project--it was not blinded, so teachers in the small classes may have been working harder to produce an effect, and parents could and did get their kids moved to the smaller classes on request.
A similar lack of effect occured in CA after they reduced class size, but it did not go down to STAR levels. Actually, aren't CA class sizes quite large?? Maybe they only did it in some areas. One theory there is that there was a paucity of qualified teachers when the sudden rush on hiring occurred after they decided to reduce class size. One thing we absolutely DO know is that teacher quality matters--a lot. The research on that is very strong.
