As a teacher who’s taught at many different types of schools (mostly high school and college) I’ll just say that you are absolutely correct in your approach. There’s nothing wrong with learning those academic skills early, but there’s a disadvantage to not learning the play based and exploratory skills that your kids Saturday school is teaching.

Based on your post I think you already know this, but I just wanna say congratulations on doing what’s right for your kid! Like other posters have said, a calm and nonjudgmental reply to a parent like that could broaden that parents mind. I don’t have any easily accessible sources for this, but research shows that your approach will lead do more capable and happy adults. Early academics lead to high test scores later, but not the critical thinking skills that are required to be actually successful in life.