Nevermind, just realised my mistake. It's because of the preposition 'do'.
Why is that? I didn't think Chodzić was a verb that required the Genitive?
The sentences about school kinda mess up with the chodzić/iść distinction. I guess this can be 'idę' in this particular example, as this may be easily understood as "I will start the school year in September" (because it's September, and Poland).
But generally, the distinction you wrote is wrong. We differentiate it this way:
to go, to walk (generally, habitually) = chodzić
to be going, to be walking (right now) = iść
to be walking (but without a purpose/direction, just walking around) = chodzić
This of course only applies for going on foot.
So it doesn't matter that from September the action will be habitual? Just when I thought I was getting the hang of it ...
Well, the main answer is 'to go' and Polish 'chodzę', both habitual, so everything's logical here.
The fact that it's about school messes things up a bit, but only in terms of accepted answers, not the main ones.
Not really, not in Polish. Generally "I go" is translated as "chodzę" and "I am going" is translated as "idę".
"We wrześniu idę do szkoły" would generally mean that I'm starting school in September. "We wrześniu chodzę do szkoły" is not the greatest sentence ever, but it means that I spend all my weekdays in September in school.