Ok... I've found the first story... it's 'Before Hogwarts' by Medico69. <https://www.fanfiction.net/s/5594409/1/Before-Hogwarts>
It's the first of a series of stories, and they go to the Peaks school in America for 1 month during the summer and have 1 year's worth of schooling per summer.
I've only gotten to chapter 11 in the first story, so i've not read about how many go, but this is the one I was looking for.
GComyn.