I think "idą" should be accepted too
While there are probably situations, where you are right, I think duolingo tries to teach that
chodzić- walk , go
iść - to be walking, to be going
So if we want to ask "do you walk to school" we use chodzić, but if we want to ask "are you walking to school (right now)" we use iść?
Yes, that's correct. Same goes for "go" - "Do you go" takes 'chodzić', "Are you going" takes 'iść'.
Okay, i did not get that one at all. What is this verb???
OK, for some reason the starred answer has been "spacerują" (They are taking a walk), but now it's back to "Oni/One chodzą".