I read this book in 9th grade, and I think that was the perfect time personally

6th grade is way too young imo for that kind of content. 11 year olds cannot typically understand racism beyond a very surface level explanation, let alone heavy racial language and major sexual assault themes. It's not that they shouldn't be hearing about it, but TKaM is like dumping Ronald McDonald in a vegan convention. It's too much for a young kid to understand, and they'll probably get scared or fail to understand the themes to begin with, which is the point of reading the book.

That said, my brother was in 4th grade when he read TKaM, and he was fine with the content. But he's a pretty mature, intelligent kid to begin with, and he still was a little unprepared for a lot of the messaging because he was like 7 or 8. He read it without my parents' knowing, and they wouldn't have let him read it if they knew because he did get a little scared and confused. 9th grade was the right age for me personally, but I know some of my classmates also couldn't handle the themes at that age