I suspect this would be somewhat unusual except in cities with a lot of highly able kids, but I could be off base. DD read books like this one (approx. 4th grade level) in 2nd grade in her GT class. They didn't ability group, though, which I didn't love.
I was rather surprised to see how high functioning these students were because I suspect most of gifted/bright children in our medium size city are not in the public school system. This particular public school at the time had the second highest reading score in the entire state and it does ability grouping for reading and math from K all the way up to 5th (except for 2nd for some odd reasons) so I think some parents who opted out of private school sought out this school for that reason. Our district allows area exception and this school is located at a highway exist that is closest to the downtown area so it makes commuting easier for many parents.
I've shared this previously but in our city, there are two well-regarded K-12 prep schools and the competition to get in at K is fierce. So children start working with a tutor (or tutors and coaches) since age 2 and as one of them also does achievement tests in addition to cognitive, parents try to get their children to function 3 to 4 grades above their age. Since both schools screen behaviors, if the child wasn't a perfect little angle with impeccable social skills and charming personality, he won't get in. So these first graders could have been rejected applicants with very disappointed tiger parents who were already aiming at 6th grade admission.
Many of these students are eventually referred to their GT program and most of them test out to be around 125 to 135. So what it is worth, MG children can read 3 grades above by 1st grade if they have been prepped for GT Pre/K admission process since birth.
