And wouldn't the experiments be more informative if the data points weren't all paying participants with rationality as a high priority?

As far as I understand that isn't the case. They do give out scholarship, so not everyone pays. I also thinks that they do testing of the techniques outside of the workshops.

Shouldn't the experiments involve trying to teach a random class of high-schoolers or something?

Doing research costs money and CFAR seems to want to fund itself through workshop fees. If they would focus on high school classes they would need a different source of funding.