The paper at https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4895486 says:

> One of the co-authors, Özge Kabakcı, a high school math teacher and former department chair of the math department at our partner Turkish high school, led the development of all session materials.

I guess it would be hard to find a high school with the sample size that you need (thousand) that will agree on collaborating. And in the US every county will have different rules and in terms of math they don't teach it in standard way.

But why Turkish not British or any other place is going to be a question no matter the location. But do you really think the results will be significantly different if it is done lets say on Vietnamese students?