I think it depends alot on the teacher.
I started kindergarten at 4. I think it was mostly to solve a daycare problem for my mom.
It was a Catholic school. I don't think i had any kind of testing. I do remeber talking to the teacher before I started though. Maybe that was their test.
Sister took me very seriously. It was a good experience. but it was a loooong time ago. and it was only a half day. but it was pretty academic from what i remeber, reading and math, we did catepillars and butterflies in science class with a worksheet recording data and stuff.
I have fond memories of Sister. I had friends in the class. it was before the era of playdates, but i got invited to birthday parties etc. so i guess it was ok socially.
